To summarize; it's a problem on Blizzard side of things,
and it's very hard for any addon using dropdowns to avoid the issue.
To verify if this is the case, try to set your focus through a keybind.
Go into Keybinds from the game options menu.
Scroll down until you find the "Focus Target" and "Target Focus" keybinds and set them up.
Now target something and press your "Focus Target" keybind (I use ctrl+f for example).
If you can set your focus that way and no action blocked errors the problem is what I said above
and you won't fix it by removing addons.
(Unless ofc you remove each and every addon that populates a dropdown menu be it in options screens or wherever with more than 16 items)
Short version: Use keybinds to set your focus and hope Blizzard will make the target frame less prone to random tainting.
Pushing this to the top because tons of people are iterating the same question about focus frame not working, when the issue has been known and reported since relatively early beta. Hopefully Blizzy will finally acknowledge it, provide a workaround or a real solution.