Old APPLE ID

I changed my APPLE ID over seven years ago on a MAC that I gave to my son after I reset it. In February or around that time APPLE sent out an update. I downloaded it as usual and moved on. When I logged into my MAC later and click on Safari I got a message saying, "to continue to use iCloud please log in. The log in box appears but the Apple ID is already input, but it's my old APPLE ID from seven years ago which I changed and even closed the email that went with it. Now I cannot use iCloud or messages without that same message appearing every time on my MAC BOOK PRO., I called support they suggested I act as though I lost the password and request a new one. I did but I get a message stating this apple id does not exist. Which it doesn't, so why does it appear every time I sign in. Then I was told NordVPN is the problem so I reached out to Nord and they have no idea what the issue is. They did, however give me step by step directions to completely delete Nord and I did still the problem continues til this day. I cannot sync my iPhone with my MAC PRO any longer because of this. Any thoughts or suggestions??