Answer
Check two common causes: first, confirm the environment's actual App Owner schema is genuinely named IFSAPP (it might be named differently in some environments, in which case IFSAPP simply isn't the right username to use at all); second, and more commonly in a multitenant Oracle setup, check whether you're accidentally connecting to the CDB (container database) instead of the specific PDB (pluggable database) the application actually runs in — the app owner schema and its data only exist inside the correct PDB, so a connection string pointed at the CDB won't see it even with otherwise-correct credentials.