Cell currently being evaluated contains a constant.

Difficult to say from a picture. What exactly is stored in A1? Is it a text string consisting of the character "3" and the character "⅝" (unicode character number 215D)?

Thoughts on fixing it:

1) How did this get entered? My first thought would be to revisit the data entry step and see if I could get the data to enter correctly at the data entry step. An entry of 3 5/8 should be automatically recognized by Excel as a mixed number. The cell value should automatically be converted to 3.625 and the cell format (assuming the cell has no other format other than General) should automatically be formatted as fraction ? ??/?? or similar. If some other behavior is in effect (maybe autocorrect is being extra "helpful" and automatically substituting the ⅝ character whenever you type 5/8), then I would try to fix that behavior.

2) If you decide that you must fix this after data entry, then I would probably use some kind of lookup table + lookup function. Enter the fraction characters into a table with the corresponding value for each (⅝ -- 0.625). Then use text functions to extract the two parts of the number, lookup the fraction, and return the corresponding value. Something like =LEFT(A1,1)+VLOOKUP(RIGHT(A1,1),lookup_table,2,FALSE)

Probably the easiest place to start understanding lookup functions is with the VLOOKUP() function: https://support.office.com/en-us/art...8-93a18ad188a1
I would expect your lookup table to look something like:

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You will probably need to pull up a good unicode list (from the Windows Character Map application or someplace like this: https://en.wikipedia.org/wiki/Number_Forms ) and go through your so you know all of the fraction characters that you need to deal with. From there, a LEFT() +RIGHT() function like I show should work (assuming all text strings are two characters long and the first character is the whole number part and the second character is the fraction character. you probably need to go through your data enough so you know all of the combinations that you need to account for.

The data was entered by someone else YEARS ago and i have no idea how they entered it.

I know it would be tedious, but it might be easier and faster to simply type over the old entries. It will probably depend on how quickly you can learn these text manipulation and lookup functions.