Why can't I install telnet on my macOS catalina with Homebrew?
Do the brew update first.

$ brew update
Fast-forwarded master to origin/master.
==&gt; Downloading https://homebrew.bintray.com/bottles-portable-ruby/portable-ruby-2.6.3.mavericks.bottle.tar.gz
################################################################################################################# 100.0%
==&gt; Pouring portable-ruby-2.6.3.mavericks.bottle.tar.gz
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Read the analytics documentation (and how to opt-out) here:
  https://docs.brew.sh/Analytics
No analytics have been recorded yet (or will be during this `brew` run).

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Updated 1 tap (homebrew/core).


Then it's ok!

$ brew install telnet
==&gt; Downloading https://homebrew.bintray.com/bottles/telnet-63.catalina.bottle.tar.gz
######################################################################## 100.0%
==&gt; Pouring telnet-63.catalina.bottle.tar.gz
  /usr/local/Cellar/telnet/63: 4 files, 138.1KB