Are you aware that you've been running a version that hasn't been getting updates since July of last year? If not, then having your OS actually get updates again might be a surprise. You'll have to update to 20.04 first, though, and then 20.10 before you can update that to 21.04. There's no direct update path (another known surprise, one might say). If you don't want to update every six months, just stick to the long-term support version (LTS, currently 20.04).