Answer
The System Parameters window itself won't display unless the user has the FND_ADMIN permission set granted — if you can't even get to the setting the error message is telling you to change, that's very likely because your account lacks FND_ADMIN specifically, not because the setting is missing or broken. Granting FND_ADMIN to the affected user resolves both problems at once: it makes System Parameters accessible, and from there the Custom Framework setting can actually be checked and enabled if needed.