Agile certainly does not replace requirements engineering. The idea that it does, is exactly the problem.

Your customer needs to know what they want. If they don't, then it is way too early to start development.

If they know only part of what they need, agile can let you start on that, while your requirements analyst (or equivalent) helps them work out the next part.

What shouldn't happen, is a lot of random sprints chasing ephemeral ideas. That's just a waste of everyone's time.