Can I pre-order the Iphone 11 at a physical Apple store?

Most assuredly. But I have known several people who weren't lucky on launch day, but were able to waltz right into a carrier or Best Buy and walk out with the phone they wanted. Actually ticked me off, because I had pre-ordered and a friend who didn't at all, got his iPhone X a couple weeks before me by walking into a less than high traffic AT&T Store and getting the phone.

All you could do is use one of their computers which would put you back in the same spot. Wait until September 20th and buy it at the store.

Without a pre-order, you can take your chances at the Apple Store on the 20th, but they likely won't have much walk-in off the street stock available on launch day. The carriers will all have iPhones on launch day, as will Best Buy, if your in the US. That said, carriers as well as Best Buy are also taking pre-orders for the new phones, so they may not have much available on the 20th either.

The Apple Stores usually have more stock than the carriers and authorized resellers and a broader selection.