This error message is telling you there is no data in your export with that field name. You can either remove the field, leaving that field empty or you can use the dropdown in the Fields in Data Source section to find the field. This error can also occur if the merge fields are too long. These fields can be shortened in Design Statements in General Ledger on the Column and Row formats. After shortening them there, be sure to also change them in the Word template to match the fields in FIMS.