Cause

This error occurs when one or more of the merge fields in your template are formatted to display the data as either number/currency or date/time formats while the corresponding data in Salesforce that is being merged to that field is a text value. The template attempts to format a string of text as a number/currency or date/time format, which will not work, so the error occurs.

What to Do

To correct the problem, locate the offending fields and update them to properly reflect their formatting as dictated in Salesforce which can be done by either a) deleting the merge field and recreating it from scratch, or b) in Word templates, remove the merge field's appearance parameter and right-click to either Toggle Field Codes back to normal or Update Field.

In Word templates, toggle field codes open on all merge fields by selecting the entire document (Ctrl + A) and right-clicking on any merge field and selecting Toggle Field Codes; these two steps can also be accomplished via keyboard shortcut Alt + F9 (on a PC) or Alt + Function + F9 (on a Mac). Once all field codes are toggled open, you can more easily look through the merge fields to locate those that have appearance parameters that are attempting to format them in a way that does not match the format of the field in Salesforce.

For example, the merge field { MERGEFIELD Name \# Currency } in a Word template would need to be updated because likely a name field in Salesforce is formatted as text while the template is attempting to format it for currency using the \# Currency appearance parameter.