I keep getting a notifications on my iMac

Please read Stop unwanted Notifications - Apple Community.

The tech recommended a malware software which didn't work either.

What tech? Please tell me you did not contact some scam artist. How did you contact him / her?

You may have been scammed into doing something unjustified and potentially damaging. Those Notifications are not malware. Installing something in an effort to get rid of them won't work and is likely to result in unrelated problems.

... none of the 4 agents could fix the issue.

😳

Whatever Apple had you install, uninstall it.

I'm sorry to hear that Apple Support was unhelpful, but I'm gratified to hear Stop unwanted Notifications worked. It's a commonly misunderstood topic, and it's not limited to Safari, or even Macs. It's a World Wide Web "feature" that was introduced a little over a year ago. The reason those worldwide wizards of smart thought it was a good idea is a mystery to me.