He assumed Ihram for Hajj, then he was prevented from entering Makkah

Question :

Whoever assumes Ihram for Hajj from the Miqat, then he travels until he comes near to Makkah, but he is prevented by the Inspection Center (i.e., the police on the road), because he is not carrying a Hajj permit card, what is the ruling?

Answer:

The ruling in this situation is that he is prevented since it is not possible for him to enter. Therefore, he slaughters an animal in the place where he is restricted from entering and he removes his Ihram. If this was to be his obligatory Hajj, he performs it later (i.e., the following year, etc.) as the first Hajj and not to make up for Hajj. If it was not the obligatory Hajj, nothing is due upon him according to the correct opinion. This is because the Prophet ﷺ did not command those who were prevented from performing 'Umrah during the expedition of Al- Hudaibiyyah to make up for that 'Umrah that they were prevented from performing. Thus, there is nothing in the Book of Allâh or the Sunnah of His Messenger ﷺ that proves the obligation of making up for the act (of Hajj or 'Umrah) for one who was prevented. Allâh said:
"But if you are prevented (from completing Hajj or 'Umrah), sacrifice a Hady (animal, i.e., a sheep, a cow or a camel) such as you can afford."

Allâh did not mention anything other than this. The 'Umrah of Al-Qadha' was called that because the Prophet ﷺ demanded that from the Quraysh, meaning that he made a pact with them to allow that, and it is not from the meaning of word Al-Qadha' that means to make up for what one has missed. And Allâh knows best.

Source:
Ash-Shaykh Ibn 'Uthaimin
Fatawa Islamiyah, Vol. 4 Page 266