Predicated on the User byline " iMac 21.5″, macOS 10.13 " which is not always accurate
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.
Does this computer appear as Listed and qualifying for an upGrade to a newer version of macOS ?
With the above information in-hand - follow this How to get old versions of macOS and verify what version this computer Qualifies to install.
Safari will open a redirect and open the Apple Store where the download can commence.
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, then 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 Sonoma is compatible with these computers
Computer Qualifying for macOS 13 Ventura
Computer Qualify for the Monterey upgrade
macOS Big Sur 11 hardware requirements
macOS Catalina 10.15 hardware requirements
Always make a Time Machine Backup before doing any upDates / upGrades
