Why can't I delete this tier?

Because something has sold on it. In the app, Delete is not offered on a tier or item with sales. On the dashboard the button is greyed and says "Tickets have sold, this type can't be deleted."

Those tickets belong to people. Deleting the thing they bought would orphan every one of them: the order, the code, the check-in, the money on your payout. So the tier stays, as history, and you close it another way.

• The admission tier on sale now: Skip to next closes it and opens the next rung.
• A group package or choose-a-price tier: open it and set its sales end to now, or use Stop ticket sales for the whole event.
• A counter item: Stop on the dashboard, Stop selling in the app's row menu.
• Anything with no sales yet: Delete works.

A tier you made by mistake and caught before anything sold deletes cleanly. That is what Delete is for.