Hey Hallix,
It seems you're having trouble getting updated from High Sierra to Catalina. Are you trying to install Catalina via the App Store? If that is giving you trouble, I would recommend using 'Internet Recovery' to update to Catalina. Before you attempt any next steps, I'd advise that you back up your data to avoid potential data loss. Additionally, Mojave and Catalina both require that your computer be of 2012 generation or newer. You can check your year model in menu > About this Mac.
How to reinstall macOS from macOS Recovery
The important part for you to follow under Step 1 is the middle option. During startup, you hold Option+Command+R. This allows you to download the newest version of macOS available, instead of just reinstalling the system you're currently on.
After loading the macOS Utilities screen, you'll want to 'Reinstall macOS', which should guide you through installing macOS Catalina 10.15.4
If these steps don't help, I would recommend that you Contact Apple Support.
Best of luck!