The AppStore Updates page only lists updates available for the OS you are running, it does not list OS Upgrades.
A 2014 mac can upgrade to any of the OS's listed in Quickposts reply.
To access them open Safari, if you use Firefox you will not be able to access the App Store.
Check with the developers what are the OS requirements for the software you would like to use.
And then decide what OS you would like to upgrade to.
In El Capitan there is the built in Photos app which you can do simple editing in, do you know if it supports
the Nikon D3500 RAW format, if not you will be able to edit the jpegs.
As for free editing apps look in the App Store.
If you are going to upgrade.
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 ******.app.
Move it to the Trash, empty the Trash.
Restart, click on the link again, the App Store should say ‘Get' or 'Download'.
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
