Mac os

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.

macOS Catalina 10.15 hardware requirements

Depending on the exact age of this computer, Catalina maybe the last version of macOS this computer Qualifies to run

macOS Mojave 10.14 hardware requirements

macOS High Sierra 10.13 hardware requirements

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 required update or upGrades to 64 Bit Versions.

You are welcome

Do come back to the Apple Support Communities ( ASC )  in the future, if / or when additional questions may arise

There has been sufficient advise / suggestions offered for the User ( you ) to make an informed and educated choice what the next course of action is required.

The suggestions have been put forth on a volunteer basis, in good faith and in the best interests of the computer.

Hi,

No, El Capitan is older than Sierra. Why would you go backwards?