I'm running v3.2.3.stable.mono.offical on Ubuntu Linux. I placed the addon folder from the zip file into my project and I see the plugins in the project settings tab. But when I try to enable either one I get:

"Unable to load addon script from path: 'res://addons/TwistedIK2/TwistedIK_PluginRoot.cs' There seems to be an error in the code, please check the syntax."

"Unable to load addon script from path: 'res://addons/TwistedIK2_EditorPlugin/Twisted_Editor_PluginRoot.cs' There seems to be an error in the code, please check the syntax."

hmm, strange. Which plug-in are you trying to enable? Twisted IK 2 or the Twisted IK 2 editor plugin? Does the console in the Godot editor show where the error in the code is, and if so, can you please share the full message from the Godot console?

Also, have you built the C# project? You may need to build the project before first use.