As I said, there seems to be a sync issue between the account on the phone and the account on the website. I don't know why and I don't know why they don't fix it. You should be able to remove devices you no longer have from the website. That was the only way I could remove my old devices.
Have you checked the Google Find hub to see if all your devices are locatable? If they are, I would not be to concerned about the failure to register notification you are getting. If your phone works, if it's showing in your account, it's all fine. It's an annoyance that you can't use Samsung Find, but as long as it can be located in Google, that's the whole point, isn't it? To be able to locate your devices.
Do try to open find again, and after you get that message, put in an error report and upload your logs.
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