The link to "Get macOS Mojave" results in a damaged link at the App Store. Is it possible to upgrade to Mojave from El Capitan, and if so, how?

Well, don't know everyone. In point of fact, millions of users have upgraded to Catalina without serious problems. Much depends on which particular model you have and how pristine the system is when you upgrade. For example, I have a 2019 27'' iMac 5K and a 2018 MacBook Pro. Both have been upgraded to Catalina from 10.15 through 10.15.6. I haven't had a problem with either one. However, if you have a 2019 16'' MBP, then you may have some problems under certain situations. Start by downloading Go64 to find all your 32-bit software and removing it. Catalina does not support 32-bit software.

Nevertheless, if you make a bootable clone of your current startup drive, then you have a quick way back should you have problems with Catalina. I also suggest looking at this:

Before You Upgrade macOS

1. Fix any problems before upgrading.
2. Run First Aid in Disk Utility while booted from your Recovery HD.
3. Backup. Make a bootable clone using CarbonCopy Cloner or use TimeMachine.
4. Be sure your startup disk has 18-20GB of free space during the installation.
5. Upgrade third-party software for compatibility. See App Compatibility Table - RoaringApps. If you have mission critical software, then DO NOT upgrade until you know that software is compatible with the version of macOS you are about to install.
6. Disconnect all third-party peripherals.
7. Download the installer which will be in the Applications folder. Quit the installer after it opens, then make a copy of the installer and save it in the Downloads folder. The one in the Applications folder will be deleted after a successful installation.
8. Installers are large files, anywhere between 5 and 9GBs. It takes time to complete. Be patient.
9. Double-click on the installer in the Applications folder. Click on the start button to begin the installation.

If your model supports Mojave, then it also supports Catalina. Thus, you need to upgrade to Catalina because Mojave is no longer available for models that support Catalina.

How to upgrade to macOS Catalina

I don't believe Mojave is not available for you if your mac can upgrade to Catalina.

The link to Mojave really does work, do not use Firefox to click on the link. Do this.

Please use Safari to access the following link do not use Firefox

How to upgrade to macOS Mojave - Apple Support

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.

Connect your mac to your router via cable rather than rely on WiFi.

Do not use your mac while it is downloading

and do not use any other devices that share the same internet connection.

If the App Store says ‘Open’ then it is detecting a previous download of the install app

go to the Applications folder locate Install macOS Mojave.app.

Move it to the Trash, empty the Trash.

Restart, click on the link again, the App Store should say ‘Get' or 'Download'.