When attempting to issue a refund in a folio, I get error message "Unable to accept payment: Credit/Refund exceeds authorized or settled amount ".

This message will display if you originally issued a Credit Card Charge on the old system and then try to issue a refund on the new MyPMS system.
The new MYPMS system will not accept a refund for an amount greater than a charged amount batched on the same system.

Therefore, we recomend that you utilize your voice authorization phone number to verbally issue a refund. If you don't know what your voice authroization number is, contact your Merchant Account provider.

A related HelpDesk article to post a credit to a folio when done 'offline' can be read at: https://support.bookingcenter.com/index.php?/Knowledgebase/Article/View/302/ to explain how to do this in MyPMS.

After you issue a refund using your voice authorization service, enter a payment refund for payment type (balance forward or write off) with the auth number in the description field of the booking folio.