google app on my phone is stuck in incognito mode

Force stop worked for mine when the toggle wasn't doing anything. Long press the Google app icon > App info > Force stop, then open it again.

Tap the incognito icon at the top right of the Google app (where your profile picture usually is) and pick turn off incognito. That should be it. If the option isn't there, update the app, and if it still won't budge clear the Google app's cache from your phone's app settings.