I can see no foundamental problem that would stop the J20 from being developed into a carrier bird. It’s size is a little large, but on par with the J15, so not a deal breaker. The main landing gears are a little forwards compared to most carrier bird, but that could be moved a little further back since they will need to do major structural strengthening work anyways. Deltas are not ideal for low speed high AoA flight as would be required for carrier ops, but the J20 is a canard delta so that doesn’t really apply because the canard addresses most of such problems with pure deltas, and as the Rafale shows, that’s not a deal breaker for carrier ops either. If the reason you heard was not one of those, then it is an obscure one indeed, so would be interesting if you could remember it.