Some of the custom controls don't work with Unity6

[battlearchers]

https://monosnap.com/file/3q87druoRi7YEcd3FRL0HbeqL4i6v0

[NullOne01]

The problem is that UNITYMVVMTOOLKIT_TEXTMESHPRO_SUPPORT is not defined.
You should add it into Player Settings like this:

[NullOne01]

I guess the problem was caused by the reason that package for Text Mesh Pro was removed. Text Mesh Pro was added into built-in com.unity.ugui package.

[NullOne01]

Moreover, seems like Unity 6.0 Preview implemented their own MVVM mechanism for UI Toolkit: https://docs.unity3d.com/6000.0/Documentation/Manual/UIE-data-binding.html
Can't find info about data binding for UGUI