What is a valid reason for not being able to perform the Passover?

Answer From The Annual Reading Schedule - Week 33 Notes For Numbers: Being defiled by a dead body or being far away on a long journey (Numbers 9:6-13) [Note: Also those physically uncircumcised are unable to eat the physical Exodus 12 lamb or goat – Exodus 12:48]. For details on defilement by a dead body, refer to the reading notes for Week 36 of the Annual Reading Schedule.