Not that urgent
Yes, this is grammatically correct. More formally, use "It's not too urgent". Note that "noncritical" and "nonurgent" (US English only) are also words in themselves, so that's an alternative too.
Not that urgent
Yes, this is grammatically correct. More formally, use "It's not too urgent". Note that "noncritical" and "nonurgent" (US English only) are also words in themselves, so that's an alternative too.