This error happens most often when you were not disconnected properly from a previous match or are experiencing active connection issues reaching the match service.
Troubleshooting is pretty limited on mobile devices due to the nature of the device but you can try the following to see if it helps the issue.
Powering down the device and then starting it back up can often solve these types of issues.
Can you switch between Cellular and a WiFi network. See if the issue persists on both network types. If the issue appears only on one connection type, then it is an issue with that connection specifically. for Cellular you would need to work with the Cell data provider. For WiFi, you would need to look at the WiFi Network.
Sometimes you can clear out bad cached network data by resetting your battle.net account password via the website and then relaunching the game.
If that doesn’t work, let’s go ahead and uninstall, then reinstall the game application. I recommend restarting the phone after uninstalling.
Now this is not a step we recommend lightly but if you are comfortable with the process you can try doing a complete factory reset of the device and then load only core apps and the client. This means setting the device up as new and not syncing previous cloud settings.