Unable to update or sign into Applications

Finally solved this issue, but it wasn't pretty. To solve it I had to go back to OS Mojave the hard way:

-Press "Shift+option+command+R" during restart to revert MacOS to original OS that it shipped with (in my case, El Capitan)

-Had to erase entire hard drive to install El Capitan, so everything on my laptop is gone (didn't affect me much as I store everything on Google Drive that is important to me)

-Installed El Capitan (1.5 hours)

-Set up laptop, then immediately googled a link for OS Mojave 10.14.6

-Another 1.5 hours installing OS Mojave

-Another hour downloading and installing all apps I used to have and downloading any important documents I need from Google Drive on my laptop.

I will not be upgrading to OS Catalina again, and will be staying at MacOS Mojave 10.14.6. It's a bummer that I had to spend all weekend trying to fix this after an official upgrade no less (this seems more like a beta release level screw up to me). Support had no idea what to do for this situation.

This is my final update. I hope no one else has to do what I did, but this is the solution I finally came up with.

Been working on this issue for literally 12 hours with no success. I'm hoping someone else comes up with a solution because otherwise I'm in big trouble Monday with no way to update or install the apps I need to do my work!

Further documenting my attempts at fixing this issue. I just reinstalled MacOS Catalina, taking an hour to do so. Upon finishing, everything is exactly the same; I still cannot sign into applications, Music, Apple TV, or anything.

Earlier I called apple support again, who were not much help, eventually just suggesting for me to reinstall the MacOS as I just did to no avail.

Very frustrating issue.

Fourth update, I really wish someone else would comment on anything that worked for them. All websites I've found via Google with the same problem, including in this forum, just ends with people still saying their Mac isn't working correctly but with no solution.

I had another chat session with Apple Support, who acknowledged many people are having this same issue on MacOS Catalina 10.15.1, however he had nothing else to offer, instead having me go through safe mode and then telling me to reinstall MacOS Catalina despite me having already done so and telling him I have done so.

So still no progress on this pressing issue, and university begins tomorrow!

This solution will not work for me as somehow no backups are showing up despite me having them. With no usb stick to boot load either I’m stuck in MacOS Catalina and have to find a solution to make things work in the current OS.

I have tried calling apple support, who only made me do the same things I had already done to no progress. Anyone else having this issue or know the solution?