Apparently, Microsoft has some concerns regarding 21H1. From their website: "We are throttling availability up over the coming weeks to ensure a reliable download experience for all, so the update may not be offered to you right away. Additionally, some devices might have a compatibility issue for which a safeguard hold is in place. In these cases, we will not offer the update until we are confident that you will have a good update experience."
It seems that they are currently rationing the general release. Since one description of 21H1 contains reference to Windows Defender, I am guessing that Defender updates are failing on my computer because I do not yet have 21H1. Given that I know virtually nothing about how Windows works, I will wait until they deem it proper to provide me with it.
