Accounts Receivable Error: This transaction is no longer valid

We are trying to process an Accounts Receivable transaction. When on the Line Items tab and attempting to enter an amount, we are getting an error "This transaction is no longer valid". We can click OK to bypass, but we're not sure why we are getting this error.

This error is usually the result of not having a default transaction type set up in Accounts Receivable. To correct, go to Accounts Receivable System Setup and add a default transaction type.