In cod mobile the network error comes when you want to start a match. This occurs when you network connectivity is low or when the cod servers are down. This is how you will have to fix it.
Check the network
Make sure that you are connected to a reliable network. If you are using the Wi-Fi then try moving closer to the router or restarting it. If you are using the cellular data then just restart your device and then the game should be working.
Update the game
Another reason for the network error can be because of the servers. The game may have updated its servers which is why you cannot play it without updating the game first. You can update the game by heading to your play store and then updating it from there. Once your game is updated close all the applications and restart your game.
Wait for the servers
The final reason can be that the servers are down from the game's end which is why you are not able to play it. The only thing you can do is wait for an hour or two till the servers are back online and by then you will be able to play the game without any interruptions.