Yes, HEADACHE is a Scrabble word!

Letter values in Scrabble US: H 4, E 1, A 1, D 2, A 1, C 3, H 4, E 1.

Is it a valid word?

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

something or someone that causes anxiety; a source of unhappiness
pain in the head caused by dilation of cerebral arteries or muscle contractions or a reaction to drugs

Also: head ache.

Not playable in Scrabble US: CEPHALALGIA.