Yes, STRENGTHS is a Scrabble word!

Letter values in Scrabble US: S 1, T 1, R 1, E 1, N 1, G 2, T 1, H 4, S 1.

Is it a valid word?

STRENGTHS is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

the property of being physically or mentally strong
capability in terms of personnel and materiel that affect the capacity to fight a war
physical energy or intensity
an asset of special worth or utility
the power to induce the taking of a course of action or the embracing of a point of view by means of argument or entreaty
the amount of energy transmitted (as by acoustic or electromagnetic radiation)
capacity to produce strong physiological or chemical effects
the condition of financial success
permanence by virtue of the power to resist stress or force

Also: military capability, military strength, military posture, strong suit, long suit, strong point, intensity level.