Userform To Populate Next Blank Row-> Not Working

As far as I can see there's nothing really wrong with your code except that I'm missnig some dimensioning.
I edited the code as follows and it does what ot shoud do

Private Sub CommandButton1_Click()
Dim ws              As Worksheet
Dim ListRow         As Long
Dim firstEmptyRow   As Range
Set ws = ActiveSheet
ListRow = ws.Cells(Rows.Count, 1).End(xlUp).Row
Set firstEmptyRow = ws.Range("A" & ListRow + 1)
With firstEmptyRow
    .Offset(0, 0).Value = TextBox1.Value 'Opp Name
    .Offset(0, 1).Value = TextBox2.Value 'Create Date
    .Offset(0, 2).Value = TextBox3.Value 'Book Date
    .Offset(0, 3).Value = TextBox4.Value 'Poject Number
    .Offset(0, 4).Value = TextBox5.Value 'Dollar Value
    .Offset(0, 5).Value = TextBox6.Value 'Secured Margin Rate
End With
End Sub

Try the attached file. I have unmerged all the merged cells. You should avoid using merged cells at all cost because they almost always create problems for macros.

Hope the OP learns from this.

Try:

Private Sub CommandButton1_Click()
    Sheets("Sheet1").Cells(Sheets("Sheet1").Rows.Count, "A").End(xlUp).Offset(1).Resize(, 6).Value = Array(TextBox1.Value, TextBox2.Value, TextBox3.Value, TextBox4.Value, TextBox5.Value, TextBox6.Value)
End Sub

Change the sheet name (in red) to suit your needs.

of course it doesn't work.
You're (I'm not aloowed to say these words out loud or in the open but you can fill in the spaces here ........) usig MERGED cells
That's omething YOU will have to mention.

First column is A, second colum is I third is M etc....

Try figuring it out yourself now

Get rid of your merged cells.

approach using structured table - eliminates the need to find range size/last row etc., - expands/contracts dynamically as you add/delete.