Application-Defined or Object-Defined error

I am trying to adapt a loop I used in a different sub to look through my sheet and find the rows where a conditional formatting rule I made has highlighted the cells yellow and send an email out. However, I am getting an application-defined or object-defined error when I try to run it. Any suggestions on how to fix this? Thank you in advance!

Sub Reminder()
    Sheets("INVOICES").Activate
    
    Dim start_cell As Range
    Dim sheet As Worksheet
    Set sheet = Worksheets("INVOICES")
    Set start_cell = sheet.Range("B1")
    Dim i As Integer
    
    Dim rng As Range
    Dim iMsg As Object
    Dim iConf As Object
    Dim Flds As Variant
    Dim TextBody As String
    Dim ApprovedTextBody As String
    Dim Finfo As String
    Dim FilterIndex As Integer
    Dim FileName As Variant
    Dim Title As String
    
    With Application
        .EnableEvents = False
        .ScreenUpdating = False
    End With
    
    Finfo = "All Files (*.*),*.*"
    Title = "E-Mail Attachment: Select file to attach."

    Set iMsg = CreateObject("CDO.Message")
    Set iConf = CreateObject("CDO.Configuration")

    iConf.Load -1    ' CDO Source Defaults
    Set Flds = iConf.Fields

 'mail server details
    With Flds
        .Item("http://schemas.microsoft.com/cdo/configuration/sendusing") = 2
        .Item("http://schemas.microsoft.com/cdo/configuration/smtpserver") = mail_server
        .Item("http://schemas.microsoft.com/cdo/configuration/smtpserverport") = 25
        .Update
    End With
    
Do While Len(start_cell.Cells(i, 1).Value) > 0
        If cells("B:U").FormatConditions(1).Formula1 Then

            TextBody = "Hello," & vbNewLine & vbNewLine & _
                "The attached invoice is still awaiting approval. Kindly review and approve it as " & _
                "soon as you get a chance. " & vbNewLine & vbNewLine & Cells(start_cell, 5).Text & _
                " Inv " & Cells(start_cell, 4).Text & vbNewLine & "JSID " & _
                Cells(start_cell, 1).Text & vbNewLine & Cells(start_cell, 9).Text & vbNewLine & _
                Cells(start_cell, 6).Text & " " & Cells(start_cell, 7).Text & vbNewLine & vbNewLine & _
                "Thank you," & vbNewLine & "cschoyer"
    
                With iMsg
                Set .Configuration = iConf
                    .To = Cells(start_cell, 12).Text + "@xyz.com"
                    .CC = "cschoyer@xyz.com"
                    .BCC = ""
                    .From = """cschoyer"" <cschoyer@xyz.com>"
                    .Subject = "Pending Approval " & Cells(start_cell, 5).Text & " Inv " & _
                            Cells(start_cell, 4).Text & " JSID " & Cells(start_cell, 1).Text
                    .TextBody = TextBody
                    .AddAttachment invoice_file_name
                    .Send
                End With
        End If
    i = i + 1
    Loop
    
    With Application
        .EnableEvents = True
        .ScreenUpdating = True
    End With
End Sub

I get the bug on the line "Do While Len(start_cell.Cells(i, 1).Value) > 0"

Thank you again for any help !

I have changed start_cell to B3 (where my data starts) but I am still getting the error. I think its because of the "If Range("B:U").FormatConditions(1).Formula1 Then" line. I am trying to find the rows that meet my conditional formatting rule, but I'm not sure how to call upon the rule. Is it possible to name conditional formatting rules?

I'm sorry. Yes, now I am getting a type mismatch error on the line: "If Range("B:U").FormatConditions(1).Formula1 Then"

This code works! Thanks for your advise.

Sub Reminder()
    Dim i As Integer
    Dim Sendmsg
    Dim rng As Range
    Dim iMsg As Object
    Dim iConf As Object
    Dim Flds As Variant
    Dim TextBody As String
    Dim ApprovedTextBody As String
    Dim Finfo As String
    Dim FilterIndex As Integer
    Dim FileName As Variant
    Dim Title As String
    
    With Application
        .EnableEvents = False
        .ScreenUpdating = False
    End With
    
    Finfo = "All Files (*.*),*.*"
    Title = "E-Mail Attachment: Select file to attach."

    Set iMsg = CreateObject("CDO.Message")
    Set iConf = CreateObject("CDO.Configuration")

    iConf.Load -1    ' CDO Source Defaults
    Set Flds = iConf.Fields

 'mail server details
    With Flds
        .Item("http://schemas.microsoft.com/cdo/configuration/sendusing") = 2
        .Item("http://schemas.microsoft.com/cdo/configuration/smtpserver") = mail_server
        .Item("http://schemas.microsoft.com/cdo/configuration/smtpserverport") = 25
        .Update
    End With

    i = 1
    Do While Worksheets("INVOICES").Cells(i, 3) <> ""
        If Worksheets("INVOICES").Cells(i, 12).Value <= Date + 7 And Worksheets( _
        "INVOICES").Cells(i, 18) = "" Then
        
            Worksheets("INVOICES").Rows(i).Interior.Color = vbYellow

            TextBody = "Hello," & vbNewLine & vbNewLine & _
                "The attached invoice is still awaiting approval. Kindly advise if this invoice is " & _
                "approved for payment as soon as you get a chance. " & vbNewLine & vbNewLine & _
                Worksheets("INVOICES").Cells(i, 6).Text & " Inv " & _
                Worksheets("INVOICES").Cells(i, 5).Text & vbNewLine & "JSID " & _
                Worksheets("INVOICES").Cells(i, 3).Text & vbNewLine & _
                Worksheets("INVOICES").Cells(i, 11).Text & vbNewLine & _
                Worksheets("INVOICES").Cells(i, 7).Text & " " & Worksheets("INVOICES").Cells(i, 8).Text & _
                vbNewLine & vbNewLine & "Thank you," & vbNewLine & "cschoyer"
    
                With iMsg
                Set .Configuration = iConf
                    .To = Worksheets("INVOICES").Cells(i, 13).Text + "@xyz.com"
                    .CC = "cschoyer@xyz.com"
                    .BCC = ""
                    .From = """cschoyer"" <cschoyer@xyz.com>"
                    .Subject = "Pending Approval " & Worksheets("INVOICES").Cells(i, 6).Text & " Inv " & _
                             Worksheets("INVOICES").Cells(i, 5).Text & " JSID " & _
                             Worksheets("INVOICES").Cells(i, 3).Text
                    .TextBody = TextBody
                    .Send
                End With
        End If
    i = i + 1
    Loop
            Sendmsg = MsgBox(Prompt:="Are you sure you're ready to send?", _
            Buttons:=vbYesNo, Title:="Warning")
            If Sendmsg = vbNo Then
            Exit Sub
            Else
    With Application
        .EnableEvents = True
        .ScreenUpdating = True
    End With
            End If
End Sub

I am working on that now. Thanks for the advise! I will post back once (if) I get it working.