From some past comments, my personal experience and what I witnessed yesterday, adding tags to a post isn't as intuitive as one would hope.
When you click on the + sign, a search box appears, with a list of choices that the system aggregates from past entries. So how are new entries created? When you start typing, if it doesn't correspond to any existing entry, the highlighted lined should appear at the bottom, (or at the top line, as I just discovered!!) "create: some-word". If you click on it, it will create the tag.
I say should because yesterday I noticed that we could not see that line on someone else's screen. This person uses the dark theme. Does that have something to do? was it because the contextual pop up bleeds below the screen? I do not know.
If anyone has that issue, I would like to know so I can investigate further!