Every time I try and catch a Pokémon, e.g. when I click the Pokémon the first time to go into the catching screen, the app will shut off. It has been like this for at least 24 hours now. How do I fix this?
Maybe you are affected by the Pokemon Go Android 0.55.0 bug.
There are a lot of reports that on this version people have problems with seeing their trainer page, incl. their buddy. Some people even reported their app crashing when they try to catch a pokemon.
You should delete the app and download it again. If that doesn't work, check if there's any corrupted data on the phone. You should also check if any other app is damaging Pokemon Go's performance. If all else fails, try to avoid playing the app or downloading it again. That will make you not care about the problem anymore, making this question useless (lol).