I had this problem as well. When I installed the gem, the output told me
Building native extensions. This could take a while...
Successfully installed bcrypt-ruby-3.0.1
so I assumed that it had, in fact, built native extensions. But when I went hunting in .../gems/bcrypt-ruby-2.1.4/ext/mri I saw that it not appear to have built anything.
I manually went in and ran
cd [path to your gems folder]/gems/bcrypt-ruby-2.1.4/ext/mri
ruby extconf.rb
make
sudo make install
This did the trick for me.
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