Why won't El Capitan install on my 2010 iMac?

I’d suspect there’s either a corruption or other oddity here, or the App Store has lost the plan (logging out of the Mac App Store and rebooting and then logging into the Mac App Store can sometimes clear that), or maybe the High Sierra installation failed.

Look around in the Applications folder for an Install macOS High Sierra app. If you find that app, confirm that your critical apps will work and that you have printer drivers and that you have external backups (all as mentioned earlier), and then invoke it.

If you don’t find the installer, click on the arrow to the right of the “installed” button text, and re-download the installer, and check your environment, and launch the installer once it’s downloaded.

What you need to know is that if the App Store notices that your Mac can install a newer OS you will be refused the download

of Wl Capitan. Your iMac can upgrade to High Sierra as mentioned above. Follow this link, How to upgrade to macOS High Sierra - Apple Support scroll down to section 4 and click on Get macOS High Sierra.

Make a full back up first. Check with the developers of any third party apps you use have updates available so they will continue to work in the new OS. Also check with the manufacturers of any printers/ scanners etc to see if they have updated drivers and software so they will continue to function in the new OS.

Tried to download High Sierra? That iMac should support be able to High Sierra.

If not, are there any anti-virus or anti-malware add-ons, or add-on cleaners or optimizers, or other similar tools installed?