F# namespace/module not defined/found within a project

File order in an F# project matters.

In this example Library.fs is referencing a module defined in Test.fs, but it is not found. Move the Test.fs file up to resolve the error.

Right click on the file and move it appropriately. In this case, Move Up.

You can also change the file order in the project file.

Moving the Test.fs reference above the Library.fs file will do the same as moving the file in Visual Studio.

Once moved, the error goes away.