meaningless rspec uninitialized constant error

I have a rails app and running on a VPS, the installation steps of Ruby, Rails, and RSpec are identical on my laptop and VPS (followed by the same instruction).
But when I want to test the app using RSpec on the VPS it gives the uninitialized constant error (something like this). the exact clone of the project runs perfectly without any errors on my laptop!
Although the tests are executing on my laptop without any problems, I tried the solutions exist on the web but didn't help me -- even I reinstalled Ruby, Rails, and RSpec on the server!

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I Need to mention that the app runs perfectly on server through the web(the app is a web-site) without any problem. Definitely this is a RSpec issue; not Ruby or Rails.

Quesions:

1. What is wrong with RSpec?
2. How can I fix this?