This error occurs when the type checker knows a token refers to a module, but is unable to find a definition for that module. For example,

import MyModule

yields

Unknown module MyModule

if there is no accompanying definition of MyModule in any loaded files.

If in PSCI, a possible fix is to load the file which has a definition for the module:

> :load src/MyProject.purs
> import MyModule

where src/MyProject.purs includes a definition for MyModule.