MacOS – iMac says that macOS High Sierra is installed but is still running Mavericks

Recently I have downloaded and installed macOS High Sierra. Downloading went well, installing went well, no problems during that process. Only when my iMac started back up I saw nothing new. It looked like it didn't install anything at all. So when I checked which OS it was running it said 10.9.5 (Mavericks).

I thought it might have been an error and I tried to reinstall the new OS. However, when I wanted to download the installer again in the Mac App Store, it said that it the OS had already been installed.

And so began my search on the internet to find a solution. I came across a post on the Apple Support Forum. It said that I should look in my applications folder to see if I still have the installer. Fortunately this was the case. Only when I ran the installer it told me that it had failed to download the necessary information from the installer. In other words: it could not run the installer.

Maybe something interesting I found online;
I came across this reply from deontaifromroseville on the forum. Here's the most important part:

Listen… I have been at this computer all day and night. Starting at 10:30a on 1/16/2018. If you see the downloaded or installed on Sierra in the App Store WITH A GRAY TAB YOU CAN NOT CLICK. It is because you have DOWNLOADED AND INSTALLED.

OPEN APPLICATION. CLICK ON LAUNCH PAD. YOUR INSTALL SCREEN OF SIERRA WILL POP UP AFTER.

However, the solution he found did not work for me. But it's still interesting because I have successfully gone through the entire installation process. Maybe it is installed but there is something else at play?

So, I still have not been able to get the latest macOS. Does anyone know more about this problem? Has anyone else experienced this same issue? Is there a solution? All information is helpful.