MAC OS High Sierra 10.13.6, 8 GB Safari Browser "outdated"

That is correct, your computer is running 10.13.6 which is getting pretty old and some websites may have decided they no longer want to support older legacy versions of Safari and other browsers. You can try another browser such as Firefox and see if the problem goes away.

This is also an indication too that your iMac well is starting to show its age. You did not mention the model year of the iMac however I suspect it may be from about 2015 or so. If it is that old (now 10 years old) then it is time to begin considering replacing the computer with a more up-to-date machine. I can tell you that any new Mac would be a huge upgrade in performance over your current machine and also you can run current versions of Mac OS.

Finally, it is wise to keep Mac OS up-to-date and to restart it about 1x per week. These actions help!