【Ruby第二章】brew install rbenvでエラーが出ています（MacでのRubyインストール）

brew install rbenv で以下のようなエラーが出ており正常にインストールができません。

==> Downloading https://homebrew.bintray.com/bottles-portable-ruby/portable-ruby-2.6.3_2.yosemite.bottle.tar.gz

######################################################################### 100.0%

==> Pouring portable-ruby-2.6.3_2.yosemite.bottle.tar.gz

==> Tapping homebrew/cask

Cloning into ‘/usr/local/Homebrew/Library/Taps/homebrew/homebrew-cask’…

remote: Enumerating objects: 12, done.

remote: Counting objects: 100% (12/12), done.

remote: Compressing objects: 100% (10/10), done.

remote: Total 552725 (delta 4), reused 3 (delta 2), pack-reused 552713

Receiving objects: 100% (552725/552725), 243.99 MiB | 7.40 MiB/s, done.

Resolving deltas: 100% (390328/390328), done.

Tapped 3847 casks (3,963 files, 261.7MB).

==> Searching for similarly named formulae…

Error: No similarly named formulae found.

Error: No available formula or cask with the name “rbenv”.

==> Searching for a previously deleted formula (in the last month)…

Error: No previously deleted formula found.

==> Searching taps on GitHub…

Error: No formulae found in taps.