Currently you can use the STK with MRTK 2.5.X, but you can't use the UPM Version from MRTK. You need to use the .unitypackage in order to be able to import the MRTK_FIX.
- Follow the Microsoft Guide to upgrade your MRTK Version
- Import the STK_MRTK_2.X_FIX (can be found in the release section)
- Thats it :)