Response Spectrum is a Linear Analysis, so you cannot use nonlinear materials.
With Base Acceleration Off, set the Acceleration to Components, then you can control the direction by using a Coordinate System in the Details. You don't have to use the Global Coordinate System.
With Base Acceleration On, you can only have a single axis and it must be a Global axis. What do you mean you can't select a Boundary Condition? That is the first line.
If you want the earthquake acceleration about an axis that is not aligned with Global, then you must rotate the geometry so that the Global axis is aligned with the desired direction.