This will be due to the account not being linked to your Google, Apple or Facebook accounts. In case of Android, please follow the steps below:
Android
Completely shut down the Battle Legion app
Restart the whole device
Open the Play Store
Search for “Google Play Games” and install/update it
Note that Google Play Games might not be available if the Google account has Family restrictions
Start Google Play Games and make sure that it logs into your Google account
Start Battle Legion
If your account still does not appear in the game, please get in touch with Player Support by using the speech bubble in the top right corner of this screen.
iOS
Restart the whole device and try loading Battle Legion
Did this help? If not, continue with the next step
Uninstall Battle Legion
Restart the device
Reinstall Battle Legion and load the game
Did this help? If not, continue with the next step
Sign out and back into your Apple account on the device
Start Battle Legion
If your account still does not appear in the game, please get in touch with Player Support by using the speech bubble in the top right corner of this screen.