The most common cause of this issue is the use of overly aggressive cleaning apps that delete necessary skin data required by FM21 Mobile. If this occurs, please try to reinstall the game from Play Store:
- Launch the Google Play Store app from your home screen or app drawer.
- Tap the search bar at the top of the screen.
- Type Football Manager 2021 Mobile.
- Tap on the installed app.
- Tap Uninstall.
- Tap OK to confirm uninstall.
- From the Google Play Store app.
- Tap Menu followed by My Apps and games followed by Library.
- Tap the app you want to install.
- Tap Install.