Today, I tried something new.
Here is an update on my Thinking... problem while using Alexa to turn on / off my TCL Roku TV. If you use the Roku app for IOS installed on my iPhone 12 to turn on the TV using the microphone button by saying "Turn on Roku", the TV turns on. Then, within 5 seconds or so, no longer, you can say "Echo, Turn off Roku" and the TV will turn off using the command issued from the Echo Dot. Then, waiting no more than 5 seconds, say "Echo, Turn on Roku" the TV will turn on. If you wait longer than 5 or so seconds, the Amazon Echo will no longer work to turn on or off the TV and it just says "Thinking..."
So, I think we can finally say that it isn't a configuration issue but a software bug.
Hope this helps. I would be interested to see if others have this same problem.