OK. So I have now had this problem for two months. Still "Thinking... but no action" issue when giving a "Alexa, Turn Off Roku" or "Alexa, Turn On Roku". I have a 2020 version of the 50S425 TCL Roku TV. Today I went and bought a 2021 version (50S431) and set up an entirely new TV with an entirely new Roku Account, disabled my Roku skill on Alexa and Enabled it with the new Roku account and TV and SAME THING! It connects to the TV as the TV says "Thinking... and spins for about 5 seconds" then the message goes away and that's it. I even tried linking the TV to a different Alexa DOT and an Alexa Fire TV cube with the same results. Both TCL and Roku acknowledge that there is a problem. TCL points the finger at Roku and Roku says that my ticket has been escalated (three times....). I even sent Roku videos of what is happening along with all the info on the software versions and hardware that I am using. Again, this seems to have happened after the 10.5.0 Roku software update around Thanksgiving.
We purchased this Alexa Enabled device for the bedroom specifically to be able to turn it On and Off with voice commands so we wouldn't have to fumble for a remote. So, taking the TV that I purchased today back and hoping that we get a Roku engineer to fix the bug in the software.