What version of Oxygen are you using (Help > About)?
Are you using the standalone edition of Oxygen or the Eclipse plugin?
I want to use a variable from the main stylesheet in the included stylesheet.
It should work if configured correctly.
Is the main stylesheet added to the list of master files and the master files support enabled?
If they are, the module will be validated in the context of the main stylesheet, so you shouldn't be seeing that error.
Do you have custom validation scenarios associated with either the main stylesheet or the module? These can interfere with the master files support.
If you validate the main stylesheet, does that validate correctly?
If the main stylesheet doesn't validate, that's the problem.