Yes, RECTIFYING is a Scrabble word!

Letter values in Scrabble US: R 1, E 1, C 3, T 1, I 1, F 4, Y 4, I 1, N 1, G 2.

Is it a valid word?

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

math: determine the length of
reduce to a fine, unmixed, or pure state; separate from extraneous matter or cleanse from impurities
bring, lead, or force to abandon a wrong or evil course of life, conduct, and adopt a right one
set straight or right
make right or correct
convert into direct current