Q: While using Safari, has anyone's browser gotten stuck on "epcassist.com System Detected Security Error"?

Is this the popup you are describing?

Close the scamware web page. Force-quit Safari if necessary.

Hold a shift key while launching Safari again.

Reset Safari: Safari menu > Reset Safari...

Make it look like this

Go to the Safari menu > Preferences > Extensions. If you see any extensions that you do not recognize or understand, simply click the Uninstall button and they will be gone.

Then, call the phone number and repeatedly hang up on them until the novelty wears off.

John has given an excellent reply, but just to clarify, this is not a virus. All it is is a tech support scam site, trying to get you to call their number. If you did, they would probably try to tell you about some dire problem with your computer - probably a "virus," but nonexistent in any case - and offer to help you with it in exchange for your credit card number and remote access to your Mac.

Do as John has suggested. If you get redirected back to this site again, take note of the site you were visiting at the time, and don't go back to that site. It's either malicious itself or has been hacked.