Unable update from 10.14.6 mojave to 10.15 catalina

Hello, dylan778.

Re: 1 - The link you have is the correct one for the full Catalina installer. I suggest that if you are using a wi-fi connection then you should attempt the download again after connecting the Mac to your router with an ethernet cable directly.

Also, make sure you are only using Safari for the download. Other browsers will probably fail.

Try the download again with ethernet cable and Safari. If you already have a previously downloaded installer in your Applications folder, please drag that to the trash, empty the trash and restart your Mac before attempting a fresh download.

Re: 2 - The update download that you linked there is NOT what can be used to upgrade from your current macOS. It's a standalone update for a Catalina 10.15 installation that one can use if the software update mechanism on the Mac doesn't work for some reason. It's unusual to have to go there.

You can make a flash stick bootable installer from this Apple document: How to create a bootable installer for macOS - Apple Support (IN). However, I would recommend you clear out more free space. 20 GB may be not enough. Try to maintain a minimum of 50 GB of free space at all times to help facilitate optimal system/application performance.

At this point, since you have no ethernet dongle for the computer and you have no access to another Mac that can download the installer for you, I'd say your best option will be to pay a visit to your friendly neighborhood Apple Store folks and see if they won't provide you a copy of the installer on a USB stick that you bring along. I've always found the people there more than willing to assist.

You can start the support process here: Genius Bar Reservations and Apple Support Options - Apple

There are factors:

1 - Like poor Wifi connection

2 - Slow wifi connection

3 - Too many other people downloading at the same time and Apple Severs are overloaded

4 - Software on the computer interfering with the download process including AntiVirus Software

5 - Running behind a VPN which may increase Packet losses and an incomplete or corrupted Installer.

6 - If via wifi too many Other devices using the wifi at the same time reducing the Bandwidth to your computer. 

And as mentioned 20 GB Empty Space on the drive is not going to work. The Full Installer size is roughly 10 GB and it requires space to download , Expand and then install.

Empty Space afterwards - it is suggested to keep at least 15% of the Drive Total Capacity as Empty Space for good operations of the computer and for additional updates to download, expand and install.