I too had this happen. I tried everything I could find on the web and had no luck. I finally broke down and called Apple Support. Apparently in my case, there was a block placed on my account. It seems to have happened at the same time I updated to Big Sur. I asked how it happened and was told it could have been my number was reported by spam or something. Oddly enough, it only happened on my macbook and my iPad, not my iPhone. So when in doubt and you find nothing that fixes the problem.. Give em a call! :)