Possible hack on my iMac and EtreCheck problem

There is a significant likelihood that you used Google to search for EtreCheck, and that you downloaded either an outdated version of it or one that has been maliciously altered. Given the resulting (fraudulent) "warnings" you encountered after using it, the latter is more likely.

Definitely follow the others' instructions for ridding your Mac of the bogus "warnings" but you will want to confirm no other potentially malicious alterations have occurred.

Do not use Google. It is by far the #1 source for Mac malware.

Whatever you downloaded, drag its app icon to the Trash. Then, download EtreCheck using the link on its legitimate home page: https://etresoft.com/en/index.html

Run the app and post its report in a reply to this Discussion. It will reveal the presence of any additional unwanted system alterations.

Instructions for posting that report are here: How to use the Add Text Feature When Posting Large Amounts of Text, i.e. an Etrecheck Report - Apple Community

While I agree that posting the EtreCheck report using the latest version may be useful, I do have to ask if the messages you are seeing are in the upper right corner of your screen seen as Notifications?

If so, that is just because you have allowed a website to send you those nefarious notifications and all you need to do is go to Safari > Settings > Websites > Notifications, and remove all websites you see there. I do see you mentioned pop-ups, but just wanted to clarify what you are seeing.

All such messages are scams.

Please see the very informative user tip below. 

Stop unwanted Notifications - Apple Community

As to the EtreCheck warning of a Beta version, this probably indicates that you are using an older version of EtreCheck.

You're all good! A website asked to send you Notifications and took that opportunity to send you those that are actually more like an advertisement intended to scare you into taking action. Once you removed the permission for the website to send you those, you will no longer see the notifications.

The Current Version of Etrecheck Application

Acquired Directly from the Developers is Version EtreCheckPro version: 6.8.12 (68084)

Restart the computer in Safe Mode

Then follow the instructions in link below

1 - Stop unwanted Notifications

2 - Phony "tech support" / "ransomware" popups and web pages