Firstly, try turning your phone on and off again. It sounds like a joke, but it legitimately solves 90% of reported issues.
Secondly, it might be a storage space issue - make sure you have as much free space as possible (Android requires a lot of space for apps)
Thirdly, it could be your device's keyboard. If you're not already, try downloading and switching to the GBoard ( https://play.google.com/store/apps/details?id=com.google.android.inputmethod.lat... ), which is guaranteed to work.
Fourthly, it could be that your device and OS version just doesn't play nicely with Ren'py, unfortunately. In that case, you can download the JoiPlay app from the Google Play Store ( https://play.google.com/store/apps/details?id=cyou.joiplay.joiplay&hl=en_GB&gl=U... ) and run the WIN/LINUX version through it.
I hope one of those suggestions fixes your issue! Sorry for the inconvenience :(