Cannot understand, why Apple-support do NOT give any answers on this subject here. It seems quite a few have the same problem. As a Windows Itunes users, facing the same problem, I managed to solve in the following way:
1) Restart machine/windows
2) ignore all Itunes related error messages (and do not try to start Itunes)
3) start Windows store
4) look for Itunes (when found it should state that the app is installed)
5) Windows store, after looking up Itunes, will start updating it - updating to Version 12.12.1.1 - helped
Restarting Itunes was successfull after compleating 1)-5)
Note: trying to start Itunes leaves someting running, which is not visible in the Windows activity management centre. Still Windows store will whine that the program cannot be installed because it it open (welcome to the world of windows), hench step 1)-2).