software-update mac OS Sierra versie 10.12.6 en hoger

An " iMac(21.5-inch,medio2010) " could upGrade to macOS 10.13 High Sierra

macOS High Sierra - Technical Specifications

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.

If there is Third Party Security Software installed - remove it.

It will interfere with both the download and installation process.

Always make a Time Machine Backup  before doing any upGrades

Pardon mijn grove taalgebruik, maar van welk besturingssysteem probeert u te upgraden naar Sierra?

Pardon my rough Dutch, but what operating system are you trying to upgrade from to Sierra?

Google translate >>  Graag gedaan !

You are welcome !