Yes, SECURITIES is a Scrabble word!

Letter values in Scrabble US: S 1, E 1, C 3, U 1, R 1, I 1, T 1, I 1, E 1, S 1.

Is it a valid word?

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

the state of being free from danger or injury
defense against financial failure; financial independence
freedom from anxiety or fear
a formal declaration that documents a fact of relevance to finance and investment; the holder has a right to receive interest or dividends
property that your creditor can claim in case you default on your obligation
a department responsible for the security of the institution's property and workers
a guarantee that an obligation will be met
an electrical device that sets off an alarm when someone tries to break in
measures taken as a precaution against theft or espionage or sabotage etc.

Also: security department, security system, security measure, security measures.