Was autopay setup prior to your due date? And did you have a balance? This is from our knowledge base regarding autopay setup.
Direct Debit (bank account)
Customers enrolling in Direct Debit and who have a balance due must make a one time payment using other self service options. The balance due will not be processed during this enrollment.
ATM Debit or Credit Card
Customers enrolling using ATM Debit or Credit Card and who have a balance due, the balance due will be deducted from the ATM Debit or Credit Card during the enrollment process.
Did you use the Direct Debit option? If so, there is an authorization period for payments via checking account. If it's showing autopay is setup on Sprint.com, it will pay your bill next month on the due date.