Because Inevitable End grants the ability to the enchanted creature, the triggered ability triggers at the beginning of the upkeep of the controller of the enchanted creature, and that player chooses a creature to sacrifice. They might not sacrifice the enchanted creature right away, but it’s certain that they’ll have to do so eventually.

Players may take actions before the triggered ability granted by Inevitable End resolves, but they can’t take any actions during a turn before that turn’s upkeep begins to try to stop the ability from triggering.