This could mean one, or all of the following issues:

The train service you have tried to book is already full.

This train is currently being used as a replacement service for the previous train, which was cancelled.

This train service has been hijacked by criminals.

This train service is delayed and you probably won’t like the new time.

This train has been rerouted away from a dangerous area.

We glanced at your payment method and you can’t afford our additional online booking fees.