If the transaction shows as having been checked out by another user. 
  1. The user who checked the transaction out will have to link the transaction. If the User is no longer with the organization and cannot link the transaction, use steps in this article for Option 3 to check in the transaction again.
OR

If the transactions show up under a tab labeled Unassigned Donations
  1. In Online Express go to Account > Integration Options
  2. Select Transaction Assignments
  3. Unmark the Enable Assignments box at the top right
  4. Click Ok