After almost a month of searching for an unlocked SG 128GB 6+, I finally got one! I'm just bumping this thread to share my experience. I tried everything I could to buy one online (T-mobile version unlocked). I live in the SF bay area, very close to Apple HQ, so my nearby store is incredibly busy. Each time I tried to purchase one online, I'd get the error message. I tried having two windows open, pre-filling my cart, none of it worked. Also, as others have stated, in-store stock is completely different from online personal pickup stock (I can explain how I know this, but trust me it's different). I had yesterday and today off, so yesterday I went to the store before it opened. I waited in line, only to find that they had none of the device I wanted in stock. The guy working the line stated that the store would "likely have some tomorrow morning." I went in today, and he was right.
I should also mention that calling the Apple phone number (say, for the store you are stalking) doesn't help. They seemed to have no clue about in-store stock, and instead simply checked the Apple website for nearby stock (which you can do yourself).
So, my advice is to forget about the Apple online purchase, and go to the store before it opens. Best of luck to everyone!