If you have set a Data validation rule your in your template spreadsheet, Form Publisher will not be able to replace correctly the markers in your template.
Therefore, the add-on can no longer duplicate your template properly, and generate files with the right information from your template.
Besides, the Data validation rules can also be automatically set when you convert an Excel file into a Google Sheet.
To relaunch Form Publisher correctly, best would be to remove your Data validation rule from your template spreadsheet by going to ‘Data > Validation … > Remove validation’:
Learn more about Data validation: https://support.google.com/docs/answer/139705?hl=en