Make sure you can compile the project through Maven (i.e. via the command line) via mvn clean compile .
If this generates errors, make sure you have a junit dependency that is mostly included in the POM.
If this does not create errors, then IntelliJ does not know where the new libraries are.
To fix this, simply go to the Maven Projects tab in the main window and click the "Create Sources and Folder Updates for All Projects" button. Then click on "Reimport All Maven Projects", which is located next to the previous button.
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