Translated by google:
>>In-store pickup window for the iPhone 18 Pro during the launch month?
How long do I have to pick up an order in-store during launch events like the iPhone 18 Pro release? I was assigned a pickup slot that I won't be able to make, and I’d like to go two days later instead.<<
You should give the Apple Store a call and explain your problem, not to be able to pick up the iPhone at the scheduled date. Some users report, that the Apple Store will hold your order for a number of days, before the order is cancelled.
Maybe you can find details about the time, if you log into your online Order Status
Have a look at this Apple Support page:
Viewing & Changing Orders - Shopping Help - Apple