installing office 2019 on Mac OS high Sierra 10.13.6

Hello, neileve.

The newest macOS version that a 2011 iMac can run is 10.13.6 High Sierra. There is no OS upgrade beyond that.

If the version of Office that you purchased requires 10.14 Mojave or newer then you're out of luck with that machine.

Since you cannot upgrade your Mac you might consider replacing your office applications with other apps like Apple’s own (free) Pages, Numbers and Keynote.  Also consider LibreOffice as a replacement.

About the Mac, unfortunately 2011 Macs can't be upgraded to a newer OS. High Sierra is the last one available for it.

About Office, MS decided to offer support only for the last 3 versions of macOS https://support.microsoft.com/en-us/office/office-2019-for-windows-and-mac-faq-a8fed43d-4c11-404a-acc3-09eb46a3a7fb. Once a new macOS is available, the third previous one gets dropped. I believe it's a very wrong move from MS and they could offer support without big technical difficulties, but it is what it is, that's their call.

You can try MS Support to see if there's any workaround.

You may also want to consider adding Windows to your Mac, or using Office 365 website to open and edit the documents.

Otherwise your formatting may not be consistent with people using Office. Even the website has some differences, but they aren't as great as third party solutions.

Another vote for LibreOffice.