I would like to “choose optional tags” for my topics. When I type a
word like “t”, a selection of existing tags appears. When I type “tag”, a message “no matches found” appears.
How can I add a new tag to my Discourse topic?
Is it perhaps limited to users with a certain Discourse trust score?
I believe only administrators or moderators can create new tags.
Which one do you want to create or are you missing?
@orschiro is right, it’s the moderators who can create tags. Otherwise it gets out of hand easily, duplicates happen and it defeats the point of organising topics. 
What tags were you thinking?
OK, that’s fine. I was thinking of using tags as in Wordpress, where you provide them as keywords to help people search for your posts. For example, for a recent post I would have chosen “Stellarium”, “OpenGL” and “Ubuntu Mate 16.04”. I will just try and make the post title do that work.
I think it’s a great idea although WordPress is one of the worst implementation of tags I have seen.
The best by far are the Stack Exchange communities.
I think that we get to the best tags over time itself.
For now, there might be still too little content to meaningfully group topics around tags.