How to compare two columns for mismatch with the reference file

I have a Reference file where Code Name and Code Number (these are not the real names of the fields but example). User selects Code Name from the form, where Code Number automatically populates the main file. I have few instances where the Code Name doesn't match Code Number and I can't figure out why. There is no one can choose another Code Number in the main file and no information is available under cell history. I would like to create some formula and then conditional formatting that would compare Code Name and Code Number based on the reference file and them highlight if they are mismatch on the main file. Here is an example. Or do you know why VLOOKUP is not working?

Reference File where each Code Name has their Code Number.

In Master file the user only selects Code Name on the form, but Code Number should be populated through VLOOKUP formula and it should look for the correct Code Number (sometimes it populates different Code Number). Here is what it looks like on the main file.


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