1. Make sure that the Expense Category is not Inactive (Navigate to Setup > Accounting > Expense Categories > Mark the Show Inactives checkbox at the bottom left part of the page).
2. If Expense Category Names are being used, the format should be the exactly the same as seen on the Expense Report Form's Category dropdown field (Navigate to Transactions > Employees > Enter Expense Reports > Expenses tab).
3. Expense Category Internal IDs can also be used instead of Names. Just make sure that in the field-mapping page, Choose Reference Type = Internal ID by clicking on the small pencil icon beside the Category field.