Macro Run Time error - Invalid procedure call or argument

I am trying to figure out why my macro won't work completely. When I reun this, I get a Run-Time error "5" - Invalid procedure call or argument. The first sheet6 changes for the "A" cells but not the B's and sheet 8 & 9 go unchanged. The 17 is referring to the number letters in the workbook name ScheduleTablet!B - Debug highlights Cell.Formula = Left(Cell.Formula, 17) & Right(Cell.Formula, Len(Cell.Formula) - 17) + 21 for sheet6

Sub ScheduleFWD()
Dim Cell As Range
Dim ShtName As String
Dim i_Len As Integer

ShtName = "ScheduleTablet!B"
i_Len = Len(ShtName)

Sheet6.Select
For Each Cell In Range("A5:A8, A11:A14, A18:A21, A24:A27, A31:A34, A37:A40, A45:A48, A51:A54, A58:A61, A64:A67, A71:A74, A77:A80, A85:A88, A91:A94, A98:A101, A104:A107, A111:A114, A117:A120, B2, B42, B82")
    Cell.Formula = Left(Cell.Formula, 17) & Right(Cell.Formula, Len(Cell.Formula) - 17) + 21
Next Cell

Sheet8.Select
For Each Cell In Range("A5:A8, A11:A14, A18:A21, A24:A27, A31:A34, A37:A40, A45:A48, A51:A54, A58:A61, A64:A67, A71:A74, A77:A80, A85:A88, A91:A94, A98:A101, A104:A107, A111:A114, A117:A120, B2, B42, B82")
    Cell.Formula = Left(Cell.Formula, 17) & Right(Cell.Formula, Len(Cell.Formula) - 17) + 21
Next Cell


Sheet9.Select
For Each Cell In Range("A5:A8, A11:A14, A18:A21, A24:A27, A31:A34, A37:A40, A45:A48, A51:A54, A58:A61, A64:A67, A71:A74, A77:A80, A85:A88, A91:A94, A98:A101, A104:A107, A111:A114, A117:A120, B2, B42, B82")
    Cell.Formula = Left(Cell.Formula, 17) & Right(Cell.Formula, Len(Cell.Formula) - 17) + 21
Next Cell
End Sub