Step 1 - Navigate Payments > Vendor credit.
Step 2 - Within the "Vendor Credit" section click on the Add button - Select a Vendor, Select status = Applied, You must enter an Amount then select an invoice to apply it to at the bottom of the CM/Vendor credit. See attached image:
Step 3 - You will need to Re-open the Vendor Credit/Credit memo and select "Edit Accounts". Apply the reversing journals.
Step 4 - (Final Step) - you will need to open the CM/Vendor Credit again and select "Edit Credit Memo". Click the checkbox "Force Sync With ERP" then hit Submit. Now your CM should synchronize with your ERP.