Ruby on Rails

I'm trying to use the rails_upgrade plugin to upgrade a 2.3.4 app to 3.2.9 and running into all kinds of things that "don't work".  I'll start with a specific problem.  Hopefully you folks can help me discover a "pattern" so I don't need to post to this group for every problem. 

The "first" problem: Based on the activemerchant documentation, the 2.3.4 plugin should be replaced by a gem.  I removed the plugin and added the gem to the Gemfile.  Bundler runs clean.  But when I then try rake rails:upgrade:check again, I get

uninitialized constant ActiveMerchant::Validateable::HashWithIndifferentAccess

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