Adding Game Assembly causes type or namespace errors

My project compiles and works perfectly, and I would like to run some tests on it. Using the Test Runner, I create a PlayMode Test Assembly Folder under Assets, and create a test script. I then create a assembly definition in a Scripts folder, and set an assembly definition reference of my Test Assembly to the assembly in the scripts folder. Doing so causes errors such as:

Assets\Scripts\MapboxCreateJunctions.cs(4,14): error CS0234: The type or namespace name 'Utils' does not exist in the namespace 'Mapbox' (are you missing an assembly reference?)

There is multiple of these type/namespace errors and I'm unsure as to why they are happening. Mapbox is a folder at Assets\Mapbox, do I need to somehow reference this in the assembly?