When I try to add a tag under “optional tags”, I find I can search for existing tags, and apply those to my post, but I can’t add a brand new tag. Nobody else has written a post about adnd-1e, and so it seems I can’t either!
Here is a video of me trying and failing. You can’t see my keystrokes but I’m pressing enter, clicking the “plus” sign, etc.
Discourse (the software used by this forum) uses a Trust Level system where users have more and more permissions as they get higher in Trust Level. A lot of admins don’t make use of it, but I do, heavily. It’s meant as an alternative to traditional moderator-committee systems—users are progressively given reponsibility to keep the forum neat, and the forum runs in sort of a pyramid fashion.
For example, the current distribution of TLs on my other forum La Locanda.
The system works so users advance automatically from TL0 to TL3 through participation, while TL4 is sort of a semi-moderator role that I have to assign manually. The Trust Levels are not shown on purpose in your public profile, to prevent it from becoming a marker of status.
TL0 is a newly-signed user. A casual user can expect to get to TL2 easily. I’ve assigned you to TL2 immediately because I already know you.
Creating new tags is reserved for TL3 users, of which I’ve manually created a handful from users that I know are used to my tagging methodology.
The right course of action for you, until you get TL3 or I assign it to you, would be to just create the thread and let us handle the tagging. As more and more games and topics are discussed, more tags will be created and this will stop being an issue.