Application Error 57121 on Worksheets("Sheet1")

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I have a command button macro that worked fine on my home computer – older Excel 97-2003 . On another computer Excel 2007 I’m getting an Application Defined  or Object Defined Error    #57121 when I get to the command

Worksheets("Sheet1").Cells(Counter, 3).Value = P1(Counter - 2)

 I added the command  Worksheets(“Sheet1”).Activate but that only caused the error at that point.