The game checks your Internet connection, and if it's not stable enough to sync or the network response time is slow (your Internet can still be fast and good at the same time), it warns you about it.
Let's try the following steps:
1 - Restart your device:
- Press and hold the Sleep/Wake button until the red slider appears.
- Drag the slider to turn your device completely off.
- After the device turns off, press and hold the Sleep/Wake button again until you see the Apple logo.
2 - Please also check if you have a stable Internet signal (for example, you are not far from your modem if you are using Wi-Fi) and open the game. You may need to reboot your modem/router. Keep in mind that sometimes networks get overloaded or malfunction, meaning that the connection may not be stable regardless of what the indicator says.
3 - Make sure you are not running out of free space on your device.
To check your free space, go to Settings of your device > General > About > Available. It's suggested you have at least 20% of free space on your device out of total. If your space is almost full, we suggest you empty some. The game can't probably download required content to continue.