I follow this tutorial: https://guides.spreecommerce.com/developer/getting_started_tutorial.html#installing-image-magick When I write
jonstark@jonstark-pc:~/
This was an issue in RubyGems 2.5.1 and it is fixed in RubyGems 2.6.3
Just update RubyGems to version 2.6.3 or greater.
gem update --system 2.6.3
Or if you use RVM (From alf's comment)
rvm rubygems 2.6.3