Why can't I buy an iPhone online from store?

An Apple event is scheduled for today. These events have historically introduced new products that are available for ordering in the store. It should be back up later today.

i want to buy iphone 13 pro 256 gb sierra blue color

You can pre-order them now. Go to www.apple.com

At this point, even if you pre-order now, delivery will likely be in later October.