1511 is a minimum requirement to purchase and download the game - the lack of such a purchase block yesterday was why some people were having download problems. The purchase block was added later in the day.
As for why you don't have 1511 offered to you in Windows Update, there are a couple of reasons.
1) If you have "Defer upgrades" selected in Windows Update settings, that would be an easy reason why. Need to disable that.
2) If it's been fewer than 30 days since you upgraded to Windows 10, you can't upgrade to 1511 yet through Windows Update. This is because once you move from RTM to 1511, you can no longer roll-back to your previous OS. You can use the Media Creation Tool from
here to upgrade in the meantime.
Please let me know which it was in your case, or if it's neither.