You probably need to buy something on your steam account to unlock the features (like 5usd or so) or you're family shared gmod while owning it iirc (it'd cause this if you don't own it, but I see you do)
https://help.steampowered.com/en/faqs/view/71D3-35C2-AD96-AA3A
#how do i fix this stupid piece of error in Crowbar?
elfin plume
Do you need to accept steam tos/eula changes on the addon page? It'd be a massive yellow 🔒 banner
Sometimes they do this after uploading, it makes you accept their latest tos before being editable