When you use SmartFill on the Account Number field, the account number does not populate in the field when you select it.
This can occur when the GL Account Index Master (GL00105) table is corrupt.
Recreate the GL00105 table. Follow the Microsoft KB titled “How to re-create the Account Index Master table (GL00105) in Microsoft Dynamics GP. There are two options available in this article.
Be sure to create a backup before following the steps in the article. After you have rebuilt the table, try your SmartFill lookup again and verify it populates correctly.