This may be an underflow error. Make sure to check class bases, unit bases, weapon bonuses and whether or not there is a “sub stat” command in any events. If any of them are negative, it’s possibly an underflow error. If it’s not an underflow error, then I’m not sure.
If none of these things work, you should try sending a report 7z here: FE_Builder_GBA -- If you have any questions, attach report7z