The phrase "conversion type to NMBR" suggests to me that the calculated field you're trying to populate is set as a numeric field, but whatever the source fields you're referencing are not numeric also. Workfront is smart enough to stop you from trying to calculate "2 + banana = ..." and will generate a mismatch error like that one. You'll see something similar if you have an existing field typed as 'text' and try to resave it as 'numeric' after data has already been entered.
To resolve, you can go back to the source fields you're trying to use in the calculation and re-type them as purely numeric. That can be a painful process if you have a large amount of existing data in the fields though, as all non-numeric values will need to be updated. Once every existing field value is actually a number, WF will allow you to update the field(s) to numeric.
I've also occasionally wondered if the conversion to numeric could be handled with expressions in the field calculation, but never had the time to test and see if that was a valid workaround or not.