If you are using modified versions of the game (Mods, APKs), or using Emulators, downloaded the game from third party website or app store then it might cause the "20MB Free Disk Space" error to occur.
Also, the culprit might be the hardware/software of your device.
In order to overcome this situation please perform the following troubleshooting steps:
1. Make sure you’ve downloaded the game from the Google Play Store. Any unofficial version of the game is bears a security risk for you, might cause you a suspension of your account and/or refusal of the support by the support team.
2. Free up device memory by closing all unused background apps.
3. Increase storage memory by uninstalling apps you no longer need. Also, try transferring (or removing) videos and pictures from your device.
If the issue still persists, please try the following:
- Go to device Settings > Apps > Google Play Store >Tap on the top right dotted lines and select Uninstall updates.
- Remove the game (don't do this if using a Guest account, upgrade your account first)
- Restart the device
- Go to the Google Play Store and install all required updates (make sure all Google services are all updated as well)
Please be aware that some devices are made for different purposes, and not all lend themselves well for gaming. The game may be unstable if your phone has a modified OS or a custom ROM.