According to the Samsung Good Lock developers, the Theme Park for Samsung Keyboard app themes has some issues. It is not working on One UI 6.
Bug emerge with Good Lock Samsung keyboard themes on One UI 6
As per the info, the issue is related to the One UI 6 color palette that is affecting Good Lock’s ability to create themes for the Samsung keyboard.
The company is said to bring the bug fix soon. For now the only solution is to disable the color palette functionality.
All you need to visit the settings, tap on “Background and style”, then “Color palette” and finally the switch to disable the functionality.