Mapping 1099 Accounts in Preferences

Its not an error message. Just like you cant change a payment without deleting it from a deposit you cannot change an expense account that is mapped to a 1099 box.

You select the vendor as eligible for a 1099, but you must also map the expense accounts to the 1099. The reason why this cannot be handled at the vendor window as you asked is because there are several types of 1099's and the information gathered is reported in different boxes based on specific guidelines. That in mind, you must indicate a vendor is eligible for a 1099 and map the expense accounts pertaining to outside vendors. 

To change an account that has been mapped to a 1099, make a list of the accounts you need to change/edit, then go to Edit>Preferences>Tax: 1099>Company Preferences> Box 7: Nonemployee Compensation (I am assuming this is where your accounts are mapped), select "multiple" and remove the check mark from the expense accounts on your list.

After you change the accounts, then go through the steps to map those to Box 7.

As a side note, make sure you are getting completed W-9s for these vendors and keeping them on file.

Please mark this post SOLVED if I answered your question : )