Can I Upgrade El Capitan to Catalina

That is compatible with Catalina.

If you intend to upgrade make sure that you understand whether you are using any 32 bit apps. If so, you will need to upgrade or replace them. See: 32-bit app compatibility with macOS High Sierra 10.13.4 and later - Apple Support

Also, before you upgrade you might want to start a new post about the problems you are having currently with Catalina to see if someone can help with that. An upgrade may not solve those problems.

Whether you can upgrade depends on your hardware. See: How to upgrade to macOS Catalina - Apple Support

If you have read the link given by FoxFifth then you would already know you can upgrade your Mac to Catalina.

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. 

Your old applications may not run on Catalina. What I recently did was partition my drive, and install Catalina on the partition to run the new application I needed. To run my new software, I just start-up on the partition with Catalina.

The problem that I am having is that my Catalina partition is no longer visible in El Capitan, which I think is related to Time Machine (I may have interrupted a backup?)

El Capitan and older Mac OS's cannot see drives formatted as APFS, so they do not appear on the Desktop.

To reboot to your Catalina, Mojave or High Sierra formatted APFS drives restart your Mac and press and hold

down the option/ alt key until the Startup Manager screen appears. Select your Catalina, Mojave or High Sierra

disk/ partition and press Return.

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Thank you so much Eau! You saved me at least a Saturday afternoon of deleting and reinstalling and .... Probably 3 to 6 hours.

Thank you and the Community!