Touch, in this context, means 'little'. So the sentence means "Sorry, it's a little late'. It's metaphorical language; a touch evokes a very light and gentle touch, suggesting that the thing in question is only slightly late.

The expression "a touch (adjective)" denotes the slight level at which a quality exists. It could be replaced with "slightly", "somewhat", or "rather".

In this context, it means "a little bit." For example, "Sorry it is a little bit too late for that."