Yes, EXTERNAL is a Scrabble word!

Letter values in Scrabble US: E 1, X 8, T 1, E 1, R 1, N 1, A 1, L 1.

Is it a valid word?

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

happening or arising or located outside or beyond some limits or especially surface
coming from the outside
from or between other countries
purely outward or superficial