Hi, I am Dave, I will help you with this.
1
Do you have any 3rd party anti-virus installed on your PC?
2
Please provide a screenshot of the error you see when you try to access App & Browser Control.
3
Click your Start Button, type regedit and hit Enter to open the Registry Editor.
Click View and make sure 'Address Bar' is turned on.
Paste this into the Address Bar at the top and hit Enter.
HKLM\SOFTWARE\Policies\Microsoft\Windows Defender
Please provide a screenshot of that Registry Editor page.