そうそう、Ubuntu 18.04.6 LTSで
$ rbenv install 3.1.1
Downloading ruby-3.1.1.tar.gz...
-> https://cache.ruby-lang.org/pub/ruby/3.1/ruby-3.1.1.tar.gz
Installing ruby-3.1.1...
Installed ruby-3.1.1 to /home/zunda/.rbenv/versions/3.1.1
$ rbenv local 3.1.1
$ ruby --version
ruby 3.1.1p18 (2022-02-18 revision 53f5fc4236) [x86_64-linux]
すんなりうまく行ったね