What I suggested was to make one or more actual variables within the MRVS mandatory. Use the checkbox on the variable definition form to do this. Your script should still be able to auto-fill the row, but when the user edits the row they will not be able to save the changes without populating the mandatory variable(s). Try this first as it is a much easier solution. If it's possible that the user doesn't even edit a row, then this probably won't work to prevent request submission.
You could also create an onSubmit script that applies to the Catalog Item, not the MRVS to check the entire value of the MRVS and alert if there are any rows with an empty value for certain variable(s), but since you are doing this in Service Portal, this script will be unable to prevent submission of the request if an empty value is found as it is happening at the same time, so the request will still get submitted. To work around this in your scenario one solution is to create another Catalog Item variable that can be hidden or read-only on the request form. This variable could be a checkbox, yes/no, or single line text. It would be blank initially and mandatory. It would get checked/populated by an onSubmit script that applies to the MRVS. So when a new row is created or edited this script would run. If every mandatory variable in the current row, and other rows in the MRVS are populated, then this new Catalog Item variable can be populated. In this way the request cannot be submitted until all of the mandatory MRVS variables in all rows are populated.
