Check if the following registry entry is set:
Key: HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Windows\System
Entry: RunAsPPL
It's an additional protection for LSASS (read more about it here), which can be enforced by group policies as shown in the screenshot, which then blocks any changes in the Defender UI (and just shows "This setting is managed by your administrator.")

If so, that's the reason you can't change it in the Defender UI, because it it forced by Group Policies.
You can delete this entry and configure it in the UI afterwards.
Alternatively, with Windows Pro or Enterprise, you can change it using gpedit:
