To prevent users from encountering the message when merging Accounts Payable type of accounts, check whether the account to be merged is used as a Default Payables Account. Follow below steps:
1. Navigate to Lists > Search > Saved Searches > New.
2. Select Vendor as Search Type.
3. Under the Criteria tab, add Default Payables Account and set to the account to be merged.
4. Preview the Search.
If there is a vendor associated with the Accounts Payable account, set the Default Payable account to blank and save the record. Go back to the account and perform the merge.