The “Could not Identify Which memory to load” is just a default message, it just shows that something went wrong and doesn’t give any information.
By what you described, my guesses would be that you either extended the class list without using EMS or you didn’t properly set up a trainee, but without a report that’s the best I can do.
Reports can be posted in the FEBuilder thread ((FE_Builder_GBA -- If you have any questions, attach report7z)), the FEU Discord’s FEBuilder channel, or NGMansion, which is the server of the creator of FEBuilder, 7743.