You have already activated rack 1.6.0, but your Gemfile requires rack 1.6.4

One experienced Rails developer highly recommends using binstubs. Have it installed by:

bundle install --binstubs

and run it via relative path:

./bin/rails server

or what is most recommended update PATH and forget about bundler issues:

export PATH="./bin:$PATH"
rails server

you need to uninstall one version of rack which is not required.

Do this please

gem uninstall rack -v 1.6.0

Reference: How to force rack to work around the usual "You have already activated rack..." bug?

Run it like this: bundle exec rails s. This will use settings from your Gemfile.lock