The docs here require updating:
Under application setup. The inform is no longer within the controller settings.
It should read:
For Unifi to adopt other devices, e.g. an Access Point, it is required to change the inform IP address. Because Unifi runs inside Docker by default it uses an IP address not accessible by other devices. To change this go to Settings > System > Override Inform Host. Tick ‘Enable’ and enter a hostname or IP address accessible by your devices so that devices can connect to the controller during adoption (devices use the inform-endpoint during adoption).