Issue updating to macOS Mojave

You can update to Mojave if you prefer Mojave to Catalina. (Note that any applications that are not 64-bit, or may have some components that are 32-bit, might not run properly under Catalina, and this is one reason why some choose to update to Mojave instead of Catalina.)

First, look in your Applications folder for an application called something like Install MacOS Mojave; put it in the trash and empty the trash.

Make a complete backup of your computer. (I usually make two backups, one a Time Machine backup, the second a "clone" type backup.)

Since you experienced an anomaly earlier, I suggest running First Aid in the Disk Utility program. When run in "live mode" from your boot disk, your computer will be unresponsive until First Aid completes, which could be 5-10 minutes. If Disk Utility finds no problems, you can go to the next step; otherwise, boot into recovery by holding CMD-R on boot, and select Disk Utility and First Aid/repair and don't proceed to the Mojave upgrade until Disk Utility shows no problems to fix.

https://support.apple.com/en-us/HT210190 This link enables you to download and install Mojave.

Because your model can upgrade to Catalina, you cannot upgrade to Mojave. Does your computer currently work? If so, what version of macOS is installed as shown in About This Mac?

How to upgrade to macOS Catalina

Learn how to upgrade to the latest macOS

You can of course upgrade to Mojave, How to upgrade to macOS Mojave - Apple Support

Go to Section 4 and click on Get macOS Mojave. This will open the App Store at the Mojave

download page. Does this page say 'Get' or 'Open'. If it is saying 'Open' the App Store is detecting

a copy of Mojave on your mac. Look in your Applications folder for Install macOS Mojave.app,

if you find it move it to the Trash and empty the Trash.

Connect your mac to your router using a cabled connection. Restart your mac,

access the App Store Mojave page again and try downloading again.

Do not use the mac whilst it is downloading and do not use other devices that share the same

internet connection.

Before upgrading make a full back up of your Mac.

Check that third party apps you use have updates available for the new OS.

https://roaringapps.com

Check that your printers/ scanners etc have updated drivers and software available for the new OS.

If you are considering upgrading to Catalina be warned that it does not support 32bit apps.

Your Mac is probably running 32bit apps and these will need to be updated to 64bit apps,

some updates may be free, some may be paid. Use this app to discover which apps you have

are 32bit, https://www.stclairsoft.com/Go64/. Once the list has been created you can then

contact the developers of the apps to see if they provide 64bit updates. 

How to upgrade to macOS Catalina – Apple Support