Yes, CAPITALS is a Scrabble word!

Letter values in Scrabble US: C 3, A 1, P 3, I 1, T 1, A 1, L 1, S 1.

Is it a valid word?

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

assets available for use in the production of further assets
wealth in the form of money or property owned by a person or business and human resources of economic value
a seat of government
one of the large alphabetic characters used as the first letter in writing or printing proper names and sometimes for emphasis; capitals were kept in the upper half of the type case and so became known as upper-case letters"
a center that is associated more than any other with some activity or product
the federal government of the United States
a book written by Karl Marx (1867) describing his economic theories
the upper part of a column that supports the entablature

Also: working capital, capital letter, upper-case letter, das kapital.

Not playable in Scrabble US: WASHINGTON.