Unable to update my iMac, what can I do?

Hi,

I have an iMac Retina 4K - 21,5 inch - late 2015 which is running on macOS High Sierra.

Although I have turned on the automatic search for updates and notify me about them, I never receive notifications.

I would like to update now.

If I go to the updates via App store - it does not show any required updates.

If I hit apple logo top left of my screen, then ' about this mac' , then ' software update' also no updates show up....

What can I do to get my iMac updated?

Thanks, Linda