If the animal is not sacrificed in the specified days
If sacrificing an animal is Compulsory (Wajib) on someone and that person fails to perform it and the days of sacrifice pass, then because sacrifice of an animal was compulsory on that person, it will not be waived. Instead, the price of an animal has to be given away in charity.
As mentioned in Durre Mukhtaar:
(Durre Mukhtaar, Vol. 5, Pg. No. 226)
Translation: If a wealthy person does not sacrifice an animal and the days of sacrifice pass, then that person should give away the price of the animal in charity.
Thus, those who could not sacrifice an animal in spite of it being compulsory on them for some reason, they should give away in charity the price of an animal and thus fulfill their Islamic responsibility.