It means that your object, sheetname, does not exist. You set the sheetnames to the activeworkbook.name which would be ThisWorkbook.Name. It is odd to name sheets with the workbook name.

You first need to move your public variables to the Module. Otherwise, you variable values will be "". After playing the two import macros, in VBE's Immediate Window, you can see the value of your two variables:
[VBA]?stringOfSheet1
VBA_Comparison.xlsm
?stringOfSheet2
VBA_Comparison.xlsm[/VBA]Debug would have shown you the same thing.

If you need a routine to see if the worksheet exists, we can add that.

Now that you know what is going on, use that knowledge to your benefit.