Hello,
The If you've been away, when should Windows require you to sign in again? setting is only available if your account has a password.
If the If you've been away, when should Windows require you to sign in again? setting is grayed out with a Windows hello is preventing some options from being shown message, it means it is not possible to change this setting while a facial recognition or fingerprint recognition Windows Hello sign-in option has been added for your account since it is assumed you want to use Windows Hello to sign in every time.
If you want to change this setting temporarily, you can temporarily disable Windows Hello biometric login.
Go to Settings > Accounts > Sign-in options and turn off Windows Hello.
Also make sure your account has a password set so that you can change the login timeout setting.
Best Regards,
Yanhong Liu
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