How to See the Stackup in Allegro Free Viewer?
Look under "Setup > Cross-section...". In Allegro Free Physical Viewer 16.6 (and 17.2, IIRC), the cross-section report lists the layers with name, material, thickness, dielectric constant, loss tangent, etc.

If you have access to Allegro Physical Viewer Plus, it shows a much prettier graphical stackup representation, but it is still the same information, just displayed differently.