Sometimes the screensaver option in Windows 10 is greyed out, so to resolve this you need follow some steps .
First your computer is not connected to a domain .
Press Start button and type Regedit.exe .
A window will open . Navigate to HKEY_CURRENT_USER\Software\Policies\Microsoft\Windows\Control Panel\Desktop
You need to delete these values in the right-pane: SCRNSAVE.EXE, ScreenSaverIsSecure, ScreenSaveTimeout
Repeat the same here HKEY_LOCAL_MACHINE\Software\Policies\Microsoft\Windows\Control Panel\Desktop only if the keys/values are present .
Restart computer. I hope this works for you ! Good luck !