Answer
FND_RUNTIME is a locked Core permission set in Apps10 specifically, which is exactly why it can't be modified via RMB/Grant — it's intentionally protected from direct changes. Make the equivalent grant on FND_ENDUSER instead, if that's the base permission set your users' permission-set structure is actually built on, or on whichever specific custom base permission set your environment uses for end users, rather than trying to modify the locked FND_RUNTIME permission set directly.