The Age of this computer will Govern what newer version of macOS it will Qualify to install. Use About this Mac from the Apple Icon on Desktop - the required Information will be there.
With the above information in-hand - follow this How to get old versions of macOS and verify what version this computer Qualifies to install. For Best results use Safari to commence the download as Others may not work.
Armed with the above required information - does this computer appear and is specifically Listed as Qualifying for any of the below versions of macOS /
If not - then this computer is too old to run anything newer that what it is running now.
Computer Qualify for the Monterey upgrade
macOS Big Sur 11 hardware requirements
macOS Catalina 10.15 hardware requirements
macOS Mojave 10.14 hardware requirements
macOS High Sierra 10.13 hardware requirements
macOS Sierra 10.12 hardware requirements
OS X El Capitan - Technical Specifications