You get this error if there is data in your Expensify Expenses that needs a mapping and the required record cannot be found in Expensify Mappings.
This is usually because the Expense contains new data that has not yet been setup. In this case you can click [OK] to have the new Expensify Mapping records created.
But it can also be because the spelling in the data has changed, and in this case you may need to edit existing Expensify Mapping records. So its important to review the list of mappings that will be created before clicking [OK].