Unfortunately looking at your data is the only solution. In order to parse your data you need to understand what type and format it is in; numbers, strings, multi-values etc.
As far as I know, verbose error messages like that are not applicable to an eval that doesn't evaluate, as the eval itself is syntactically correct. This means the data is either not there (empty field, wrong field name) or the data is not the correct format (string when trying to add numbers.)
Try using commands such as :
| fieldsummary (to get a snapshot of what your data looks like for each field)
| metadata (to get an idea of what types of data is associated with what indexes/sourcetypes/sources)
| eval numberornot = if(isNum(fieldnamehere),1,0) | stats count by numberornot (tells you how many of your events have a number in that fieldnamehere field, if all of them, then you are probably safe to do calculations, if it is a mixture then build case or if statements to cover the possible inputs
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