$ gem install bcrypt -v '3.1.16' --source 'https://rubygems.org/'
Building native extensions. This could take a while...
ERROR:  Error installing bcrypt:
        ERROR: Failed to build gem native extension.

    current directory: /home/$USER/.rvm/gems/ruby-3.0.2/gems/bcrypt-3.1.16/ext/mri
/home/$USER/.rvm/rubies/ruby-3.0.2/bin/ruby -I /home/$USER/.rvm/rubies/ruby-3.0.2/lib/ruby/3.0.0 -r ./siteconf20210811-1329082-xzlqe3.rb extconf.rb
creating Makefile

current directory: /home/$USER/.rvm/gems/ruby-3.0.2/gems/bcrypt-3.1.16/ext/mri
make DESTDIR\= clean

current directory: /home/$USER/.rvm/gems/ruby-3.0.2/gems/bcrypt-3.1.16/ext/mri
make DESTDIR\=
compiling bcrypt_ext.c
compiling crypt_blowfish.c
gcc  -D__SKIP_GNU     -c -o x86.o x86.S
compiling crypt_gensalt.c
compiling wrapper.c
wrapper.c:182:60: warning: ‘struct crypt_data’ declared inside parameter list will not be visible outside of this definition or declaration
  182 | char *crypt_r(const char *key, const char *setting, struct crypt_data *data)
      |                                                            ^~~~~~~~~~
cc1: warning: unrecognized command line option ‘-Wno-self-assign’
cc1: warning: unrecognized command line option ‘-Wno-parentheses-equality’
cc1: warning: unrecognized command line option ‘-Wno-constant-logical-operand’
linking shared-object bcrypt_ext.so

current directory: /home/$USER/.rvm/gems/ruby-3.0.2/gems/bcrypt-3.1.16/ext/mri
make DESTDIR\= install
make: /usr/bin/mkdir: Command not found
make: *** [Makefile:202: .sitearchdir.time] Error 127

make install failed, exit code 2

Gem files will remain installed in /home/$USER/.rvm/gems/ruby-3.0.2/gems/bcrypt-3.1.16 for inspection.
Results logged to /home/$USER/.rvm/gems/ruby-3.0.2/extensions/x86_64-linux/3.0.0/bcrypt-3.1.16/gem_make.out