Depending on what the Mac is running now, and its build year date; there should be
some upgrade potential beyond the macOS where it was last left waiting. Also a few
other things may need checked such as battery (leakage/swelling?) as those things
age-out due to chemical depreciation over time.
This site has macOS 'system Installer downloads' & ways to identify one(s) that work.
To know your Mac identification, you could use 'About this Mac' from Apple Menu.
We don't post 'serial numbers' here, use references for your sake, offline.
A page you could enter a serial number into https://checkcoverage.apple.com/
should tell you what vintage and identifiers fit your hardware.
• How to get old versions of macOS - Apple Support
https://support.apple.com/en-us/HT211683
Sometimes to upgrade to a newer macOS, an in-between version may need to be
installed first; these leap-frog ahead (with those, they may provide firmware update.)
Then later full systems could be upgraded. Otherwise some won't work.
For example, some hardware build years may need an installed El Capitan
10.11.6, before later (10.12. +) could work. Some users found this necessary.