Recently with the roblox group update, the API for changing a member rank has seemed to change. It may just be a simple error with the program itself or a change in roblox’s api. Here is the snippet that usually does it:
$promote_user = curl_init("http://www.roblox.com/groups/api/change-member-rank?groupId=$group_id&newRoleSetId=$new_role_set_id&targetUserId=$target_user_id%22);
If you know PHP and can assist in this issue, it’d be great if you could respond on this post or if you have Discord, add me (Bark#4492).