Did you try clicking somewhere in the blank space of the dialog? That should bring up a "done with action" button and then clicking that will bring up the done button.
Your best bet might be to remove this rule and re-create it. Something probably got messed up and RM thinks you're in a state that you're not in. If you can find a reliable way to reproduce this problem, staff would probably be interested in (and capable of) fixing it, but otherwise odd things like this happen sometimes, and starting over is usually the best "fix."
If you don't want to try that, I'm curious if "Cancel this action" will help in your case and allow you to add an action as you expect the next time you try. No guarantee, but that's probably all you can try (besides trying the same in a different browser or clearing cache, I suppose) besides the above.
This happened to me a couple of times.
It happens most often after adding and deleting some condition statements and eventually will get it into some wrong state.
The only way to get it out of the wrong state is to remove the rule
I've looked into my code for the Aeotec eTRV (I assume that's the TRV driver we are talking about) and my attributes are all either number or string.
On looking at it more closely they were number or String (capitalization) - I've changed them to be Number and String - as that's what @bravenel had in his post.