Make sure your MONTHYEAR* fields are dates. Text values can't be subtracted from each other in a meaningful way.
Then you can use a Select in Field action assigned to the OnSelect trigger of the MONTHYEAR field to select the previous month in the MONTHYEAR_QVD field with a search string like
The date format YYYYMM assumes that your MONTHYEAR* fields contain dates with a format YYYYMM. If the date format is something else then you need to adjust the format string in the Date function accordingly.