A.K.A. "Foire Aux Questions" (FAQ) in French. (useless fact but why not)
I'm sorry, but I can't help if you just send me this. At least send me a screenshot or a text transcription of what's on the screen, if a terminal (a black window) is opened, please report what's written on it.
You wanna try to get your GD savefiles back, yes or not? It's the only way, as far as I know.
Use Android or Linux instead. I have no idea where are the savefiles saved. Good luck.
If you have the savefiles, use Wine to get a Windows environement (to get support fixing for Windows/Android-like savefiles),
or Darling to get a Darwin (MacOS) environement (to get support for fixing MacOS/iOS-like savefiles) Version 2 now automatically selects the right format even if you're not on the right OS and fixes it in the format it originally was. You won't be able to convert from a format to another using this tool. (Maybe I'll make a future 2.1 update where I will add this functionality...).
Firstly, check is the guarentee still available for your device! Because maybe it's not, so you won't cancel anything. See Apple's website: Apple Check Coverage. Also, if it's not a guarentee directly given by Apple (example: a guarentee given by your local tech shop), you have to check their terms. Maybe they allow jailbreaking under certain terms, or maybe they allow it if you pay them to do it. I don't know everything, however, searching is the most precious power in the universe. Finding is the second one.
If you cannot open Geometry Dash, then try this. It is not guarenteed to work, but it may in 50% of time. It fixes some weird data put by Geometry Dash in it for some reason and that the game can't handle, because we all know that RobTop is the most unlogical programmer that ever existed. It works on official versions from Google Play Store, Apple Store/iTunes, Microsoft Store and Steam, but also for cracked versions and other custom versions such as unofficial versions (ex: unofficial 2.2s) or GDPSes.