I noticed when I was deleting my old adverts for groups I have already left, Roblox would not allow me to delete them saying that I have “insufficient permissions”. This bug has happened with 2 of my old adverts where I have left the group that the advert is linked to. The only way to get around this issue is to rejoin the group and hope that the owner allows your roll to advertise the group, this can be impossible if the owner has left the group and no one is owning it with join requests on.
An error message pops up like this
I have only noticed this bug today but I found an older post from 2015 where they have seemed to have the same problem as me.