How you get that error you get would have helped. Just looking at the code what struck to me is that you are updating the cell in the code. This would put you in a loop. You should disable the event before updating the cell and then enable it back
Some thing like this
Private Sub Worksheet_Change(ByVal Target As Range)
If Intersect(Target, Range("T12:T111")) Is Nothing Then Exit Sub
Set MR = Range("T12:T111")
Application.EnableEvents = False
For Each cell In MR
If cell.Value = 4 And cell.Offset(0, 1).Value <> "C" Then
Range(cell.Offset(0, -14), cell).Copy
Sheets("Cash flow").Range("E" & Rows.Count).End(xlUp).Offset(1, 0).PasteSpecial
cell.Offset(0, 1).Value = "C"
cell.Offset(0, 1).Font.ColorIndex = 2
Application.EnableEvents = True
How ever this may not resolve your issue as there is not much information. If you still have issue, post a sample book with the macro and sample data on some shared site and post back the link to the file back here. Also do mention what error you get and what you do to get that issue.