Nested For Each loops

You need to specify the worksheet when you referring to a range. Give this a try

Sub test()

Dim ws As Worksheet
For Each ws In ActiveWorkbook.Worksheets
    If IsEmpty(ws.Range("a1").Value) = False Then 'If range A1 is /not/ empty, then perform these loops
        
        Dim c As Range
        ws.Range("d3:x115").Name = "Rng"
            For Each c In ws.Range("Rng")
                If c.Interior.Color = RGB(166, 77, 121) Then 'If it’s colored then
                    If c Like "*#*" Then c.Offset(, 5) = c + 1 'check if has a date entered
                        If c.Offset(, 5).Interior.ColorIndex = xlNone Then c.Offset(28, -20) = c + 3 'if no fill then go to next week
                    End If
                
                Next
    ws.Range("d143").ClearContents
    On Error Resume Next 'If range A1 /is/ empty, then go to the next worksheet
End If
Next ws

End Sub

Maybe ...

Sub test()
  Dim wks           As Worksheet
  Dim cell          As Range

  For Each wks In Worksheets
    If Not IsEmpty(wks.Range("A1").Value) Then
      For Each cell In wks.Range("D3:X115")
        With cell
          If .Interior.Color = RGB(166, 77, 121) Then
            If .Value Like "*#*" Then .Offset(, 5).Value = .Value + 1
            If .Offset(, 5).Interior.ColorIndex = xlColorIndexNone Then .Offset(28, -20).Value = .Value + 3
          End If
        End With
      Next cell
      wks.Range("D143").ClearContents
    End If
  Next wks
End Sub