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.
Safari will open a redirect and open the Apple Store where the download can commence.
Would suggest doing a Step Up to at last High Sierra macOS 10.13 before attempting to go any further. Once that is achieved, you may proceed to upGrade again and eventually reach as high as Big Sur macOS 11.6.8 ( current stable version )
macOS Big Sur 11 hardware requirements Big Sur the the last version of macOS this computer qualifies to run
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
Tips Big Sir Upgrade Released Nov 2020
Make Time Machine Backup is very useful and can be used to Revert to Previous Working macOS
Notation - Mojave is the Last version to support 32 bit applications / extensions and Drivers. Catalina, Big Sur & Monterey supports only 64 bit and there is no workaround.
Suggest using Go64 to find 32 bit application before attempting the upGrade. Some or many of the Application already installed may require an update or upGrades to 64 Bit Version
