A syntax error means that there's a problem with the way the line of
code was written, and it should change to red text when you move the
cursor to a different line of code.
Maybe there's a space before one of the periods, or an extra character
somewhere.
Also, the code, as written could give incorrect results if Sheet1 is not
selected when the workbook opens. It would add 1 to the value in cell A1
of the active sheet. To correct this, you could use the following code:
Sub Workbook_Open()
Sheets("Sheet1").Range("A1") = _
Sheets("Sheet1").Range("A1").Value + 1
End Sub