hello @vundeq
Roblox Disabled the endpoint
More information here: Attention group owners: Critical group security flaw with ranking users - #4 by Hooksmith
That means, That all setrank functions of the group admin is now not working.
Also Please This is an old topic, I don’t think it’s a good thing to take it back for this problem
you’d better create a new topic or just watch this: Unable to Rank Someone in Group Bug
Just wait roblox to fix the problem.