How do I delete. " this setting is managed by your administrator"?

Google provides instructions for connecting your Android phone to your Chromebook. Note that you have to be logged into the same Google account on both devices. See the page titled "Connect your Android phone to your Chromebook - Chromebook Help".

If you're being restricted by an Administrative policy, you'll need to remove the Chromebook from the managed network. Outside of contacting the network administrator, the only other way to do it is to wipe it completely, assuming the “forced re-enrollment” setting is not enabled (which it is by default). It's also possible that Developer Mode that's used to wipe the computer is blocked by your administrator. See "How to Reset a Managed Chromebook (School, Work, or Enterprise) - Complete Tutorial (2022)" for the complete set of instructions.