Make sure that you also have a Gemfile.lock, which is generated when you do a bundle install or bundle update in the directory (where the Gemfile is).
Also make sure that the Gemfile and Gemfile.lock are in the same directory, and are in the directory where you run the fastlane step from.
If you need more help with the Gemfile based setup please copy paste the build’s URL, without that it’s hard to tell anything more specific 
Alternatively you can use a Script step to install the fastlane version you want, as described at: How to use a specific version of fastlane?