It happened to me as well but what I did was:
1. I went to the registry and navigate to "Computer\HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Windows\DataCollection"
2. Then Deleted the Value "AllowTelemetry"
3. Then Restarted my Machine
4. From the Run Dialog box (Windows + R), type "Services.msc" to open the services
5. Search for "Connected User Experiences and Telemetry" and see if that service is running or not.
6. If it isn't then:
6.1. Right-click on it
6.2. Click on "Properties" from the contextual menu
6.3. And in the Dialog box, you should be in the "General" Tab, so look for "Startup type" Dropdown menu
6.4. Within the menu, select "Automatic"
6.5. Click on Apply then OK.
7. Restart the computer and go to the settings again, you should be able to activate your "Optional Diagnostic" from now.
Blessings
