What does the Buddha mean when he says I can't cross the world and I won't become a Buddha?

Use your head. Since the Buddha said it, it means becoming a Buddha. How is it possible to come up with a sentence "I won't become a Buddha"? This is a joke.

Many practitioners who have not become Buddhas, such as Bodhisattva and Lohan, have made this wish. If you don't spend your whole life, you won't become a Buddha. Still use this bodhisattva to measure all beings. Like ksitigarbha bodhisattva, there are endless beings, and ksitigarbha bodhisattva will never die.