Too many times we as developers see rank/team/job/group names interchanged.
For sake of simplicity, a "group" membership has nothing to do with rank/job/team when we ask here.
"group" level access is what servers use to determine how much access you have to server commands. ULX is intended as a server administration mod.
superadmin is a group.
admin is a group.
user is a group.
Those are the three that Gmod understands, and expects non-custom-group scripts to follow.
My bet is that you aren't high enough group access that your mod/addon/whatever sees you with enough access to touch others.
Usually, that requires superadmin.