Unable to upgrade my iMAC from Mojave to Catalina

You can boot into Safe Boot mode, by holding the shift key until the progress bar appears. This will be slower as there is System house-keeping in progress. Try the upgrade process from there, or reboot normally after Safe Boot mode, and try the upgrade again. Hopefully, you have a final Mojave Time Machine backup before you proceed with an upgrade.

Do not rely on WiFi, a cabled connection will be more reliable.

I have 2019 iMac 27 inch with Retina 5K display 3.7 GHz 6-core 9th generation Intel Core i5 processor, Turbo Boost, Radeon Pro 580X with s8GB of GDDR5 memory, 3 TB fusion Drive storage bought new directly from Apple in May 2019. I've had unbelievable continual problems with this iMac, been hours and hours on phone with Apple techs, finally was told I needed to wipe entire disc clean and reinstall which was done with help of Mac tech over phone; continual little problems and more tech support unusal things I've had Macs for decades never so many problems, ever.

The final straw is it WILL NOT load Catalina - at least 4 technicians from Apple 2 or 3 of them senior techs failed to load it no matter what, they spent hours examining my computer finally last one said we need to wipe your disc clean and reinstall and I said I think I have a lemon, the tech said you better do something before your warranty expires in May 2020. And SO I'm going to do something, the senior tech had no words of comfort. I'm a publisher, on deadline, need to work continually getting publications to press; I have PTSD from all breakdowns and hours and hours of tech support that never solves problem and NOW final straw this brand new iMac 2019 cannot load Apple's latest OS Software. Certainly something is VERY VERY wrong. I would like to know how many others with 2019 iMac are having serious problems of this nature. And if ANYONE has any help...keeping in mind the several senior Apple techs who already tried everything under the sun to load Catalina and have failed...last one said I should wipe my entire disc clean and reinstall again...that is a process with huge undertaking and I've only had to do it once in my life and I've used Macs since 1980s. I should have asked for another iMac after my 10th or 15th call to apple support... never dreamed they'd be selling lemons.

We discussed that and I have ethernet connection too for my printer to computer I spent so many hours with different techs trying to load Catalina I think at one point I reconnected my computer directly to modem and that didn't work either.

I try to take notes during these calls but after a while, and I spent several days in a row I could not afford away from my work trying to get Catalina installed at a time when I could adjust to new OS before it was too close to next deadline; never want to deal with new OS and deadline issues at same time. We did everything. my computer sits 10 feet from modem and wireless is always highest it can be, I have full on business internet connectivity.

They rebooted in safe mode didn't work.