Excellent point by @VikingOSX and souls have realized your question was appearing in macOS 11 Big Sur but really this computer is running an older version of macOS
Example ruling macOS 10.13 High Sierra and refer to image below
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.
Does this computer appear as Listed and qualifying for an upGrade to a newer version of macOS ?
If Yes, suggest having at least 50 GB of Empty Space on the drive.
It will allow the Full Version ( about 12.xx GB ) to download, expand and then commence to install.
If there is Third Party Security Software installed - remove it.
It will interfere with both the download and installation process.
The below links will help identify which version of macOS this computer may Qualify to run.
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
Make Time Machine Backup before doing any upGrades
Notation - Mojave is the Last version to support 32 bit applications / extensions and Drivers. Catalina, Big Sur, Monterey, Ventura 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
