Looking at the low number (3) in the banner I think that may be a Basic plan site? (it’s normally 50) If so, then the setting for bootstrap mode is a hidden setting on the Basic plan and can’t be tweaked like other tiers. But the banner is only visible to staff, and will clear within 24 hours after you hit the target number.
Just for the record @Jagster, the admin setting search does work if you use the underscore version of the setting name (as long as the site setting isn’t a hidden one)