Problem with running NUnit tests


I have created a build configuration, which runs tests in NUnit framework.
In these tests we use ConfigurationManager class to get values from App.config file.
When I run these tests manually, everything works.
However, when I run these tests using TeamCity, either no value can be found in app.config file or the app.config file is not recognized. I any case, I get an error" "the value cannot be null". 
We get the values from app.config in this way:
Can someone please tell what the problem is?

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