You cannot easily revert to a release version of Windows 11. I opted out of the program at the start of the year, but getting out of the system is not so easy. Also at no point was there any notification prior to my purchase that this would be an issue. It was literally not stated anywhere, I looked.
Additionally the functionality (transparent Taskbar) LITERALLY WORKS PERFECTLY IN EVERY OTHER APP PROVIDNG THIS FUNCTIONALITY regardless of my Windows 11 version number, and lets be clear here, I am not using a beta, or unsupported version. I am literally only getting the next production version of Windows 11 early. The RCs thus far have had no issues, and are no different from the production versions when they release. Start 11 breaks the other apps with Transparency, because you've disabled it. I also cannot use these other apps with Start 11 enabled, because Start 11 breaks them too.
Why can you not enable the feature? Because from all I can see the only reason it's not working is because you've disabled it.
What build number is your Windows?
If you get those messages and you are using a current build of Start11 (1.31 or 1.32) then Start11 has detected your system has the new Taskbar that Microsoft are still working on. This does not work with transparency adjustment which is why the setting is disabled. I would certainly check you are using the latest Start11 however as I think you are the only user reporting this on Insider Preview (vs beta\dev)
Regrettably the use of Windows insider builds means you may experience issues due to Microsoft changes. If you are on release preview then I believe thats not due for release for months so it shouldn't be unexpected that we will expect to use that time ourselves for development. This is one of the risks of using unreleased software.
One curious thing is if you opted out early in the year then I would have expected you to have ended up back on the release build back in October. Are you 100% sure you were initially on Release Preview rather than beta?