Can I find the loss tangent of a substrate if I know the thickness of substrate and its relative permittivity?
No. Relative permittivity (a real number the way you specified it) and \$\tan\delta\$ are two different properties of a material. You can have a low-loss and a high-loss material with the exact same real part of the permittivity.

You should probably re-read your material; you'll find something about complex-valued wavenumbers, and you'll see why permittivity and loss are two orthogonal things.