It looks like you're encountering FGO Error Code 99. This error typically appears when the game temporarily locks your IP address due to unusual network activity, like frequently switching the game on and off. This is a protective measure to prevent server overload and ensure a smooth experience for everyone.
Credits to that one lad from other platform...
Wait It Out: The lock is usually temporary, lasting about half an hour. Try not to repeatedly log in during this time, as it might extend the lock.
Restart Your Device
Switch to a Different Network
Use a VPN
Clear Cache and Data (Android)
Reinstall the Game