As you said, that the error occurs after the action of new Button, it may be because SouceData/Report_ATA is Empty…
Have you double checked the value in your microflow ?
Try add a conditional check in your Change Variable:
if ($SouceData != empty and $SouceData/Report_ATA != empty) then formatDateTime($SouceData/Report_ATA, 'yyyy.MM') else ''
This usually occurs if one of the attributes you try to format is empty. In this case Report_ATA is probably empty, so the microflow throws an error.
In this case I would recommend using the debugger to check if Report_ATA is indeed empty when you try to do the calculation.
Hope this helps!