Hello! I'm having the same issues.
I have the Mono 64-bit version of Godot downloaded, and the "Mono 64-bit (no GTK#)" installed from here: https://www.mono-project.com/download/stable/ as well as the .NET Core SDK 3.1 x64 version.
I created a new project, added a node, and added an empty .cs file to it. I clicked build and the build failed, so I went to Editor > Editor Settings > Mono > Builds > and switched "Build Tool" to "MS Build (Mono)". I tried running the project again, and this time it succeeded.
I added the "addons" folder from the "Free_Twisted_IK_2.zip" to the root of "Development_Example_Project" and opened the project up. I opened up "3D_Test_Scene_1.tscn" and clicked "Play Scene" (the project is using "MSBuild (Mono)") and the scene was able to build, but stops after a few seconds and throws these errors:
"""
can_instance: Cannot instance script because the project assembly is not loaded. Script: 'res://addons/TwistedIK2/IK_3D/Twisted_ModifierLookAt3D.cs'.
<C++ Error> Method failed. Returning: __null
<C++ Source> modules/mono/csharp_script.cpp:2907 @ can_instance()
"""
When navigating to Project > Project Settings > Plugins the "TwistedIK2" and "TwistedIK2 Editor Plugin" are visible, but neither is enabled. Clicking "Enable" on either of them gives the error:
"""
Unable to load addon script from path: 'res://addons/TwistedIK2/TwistedIK_PluginRoot.cs'. This might be due to a code error in that script.
Disabling the addon at 'res://addons/TwistedIK2/plugin.cfg' to prevent further errors.
"""
The paid addon and "Twisted_IK_2_Showcase_Project" have the same issues described above.