Trying to block a group via the website, but doesn't work

{"error":{"message":"You must leave a group before blocking it․","status_code":403}}

Any updates?
The leave request clearly states I’m not a member, yet I cannot block it.

{"error":{"message":"You're not a member․","status_code":403}}

Sounds like you might want to contact VRChat support, they should be able to force you out of a group.