Your mac being a 2015 model can upgrade to any macOS from Sierra, High Sierra, Mojave and Catalina,
it will probably be able to upgrade to Big Sur when it is released in the near future.
How much RAM does your mac have installed, Mojave and Catalina benefit from having
8GBs of RAM minimum, the others are OK on 4GBs minimum.
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.
With the introduction of macOS High Sierra Apple introduced the new APFS file system.
When upgrading to any of the newer OS's from an older one like El Capitan the installation
will convert the internal SSD to APFS, in some cases others have experienced that if doing this
from older OS's like El Capitan or Yosemite the installation can get stuck at the conversion phase.
With that in mind I would recommend upgrading first to macOS High Sierra and once you have got
that installed and working you can consider upgrading further either to Mojave or Catalina.
Mojave being my favourite of the two.
Please use Safari to access the following links do not use Firefox
How to upgrade to macOS Mojave - Apple Support
How to upgrade to macOS High Sierra - 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 ******.app.
Move it to the Trash, empty the Trash.
Restart, click on the link again, the App Store should say ‘Get' or 'Download'.
How to upgrade to macOS Catalina – Apple Support