Yes, WEAKER is a Scrabble word!

Letter values in Scrabble US: W 4, E 1, A 1, K 5, E 1, R 1.

Is it a valid word?

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

wanting in physical strength
overly diluted; thin and insipid
(used of vowels or syllables) pronounced with little or no stress
wanting in moral strength, courage, or will; having the attributes of man as opposed to e.g. divine beings
tending downward in price
deficient or lacking in some skill
lacking bodily or muscular strength or vitality
(used of verbs) having standard (or regular) inflection
not having authority, political strength, or governing power
deficient in magnitude; barely perceptible; lacking clarity or brightness or loudness etc
likely to fail under stress or pressure
deficient in intelligence or mental power

Not playable in Scrabble US: DEBILE.