Why doesn't printing work from a specific screen?

Most likely you have not given a permission for the unTill Air app to search devices (printers) in the network causing a constant printer error.

If you receive constant printing errors in the app and already have checked that IP address is correct and your screen and printer are connected to the same Wi-Fi network, check also in the settings of your device that the unTill Air has been granted the 'Local network' rights which allow to connect to the printer.

Go to the settings of your phone/tablet and in the apps section scroll to unTill Air, then among the permissions for this app, make sure that 'Local network' is turned on.