The flight to Toronto, Canada, is fully booked, but we'll surely come  get/be in touch with you if there are any __.
(A) cancelled
(B)cancelling
(C)cancel
(D)cancellations

(D) sounds perfectly OK to me to complete the sentence above and I know that it's the only answer. True.

But I don't know how I can explain why (C)-"cancel" is ruled out. Grammatically the word 'cancel' can used as a noun and it also means "cancellations" according to the dictionary.

Can you please quote the entry in your dictionary that indicates this can be a noun meaning 'cancellations'? Perhaps you have misinterpreted it.

My dictionary quotes two cases where 'cancel' can be a noun.

1, an order revoking a previous one.

2. the cancellation of a postage stamp.

I've never heard #1, and #2 is obviously a very specifc usage. Neither seems relevant to your example.

Finally, consider this. The phrase '. . . if there re any _____' obviously needs to be followed by a plural word.  Option D is the only plural choice available.

Best wishes, Clive