Some price types may have a limited number of tickets available. The reason why you are unable to select tickets here, or you receive an error message, may be that the number of tickets you have selected exceeds the number available for the price type(s) in question.

This means that there are no more tickets available for the price type you have chosen.

An example of this could be festivals that post "Early-bird tickets". Here, a limited number of tickets will be available at a discounted price. Remaining tickets will be placed at ordinary price, or other price types. It is up to each individual organizer to choose which price types they wish to use for their event.