This is my first bank reconciliation after data migration from Sage but my starting bank balance is zero.

Welcome to the QuickBooks Community, bookkeeper-galep.

Thanks for reaching out to us here. QuickBooks Online is a great software that can help keep your books balanced. Reconciliation is a great way to ensure your books are accurate. I recommend reviewing this article on how to fix issues the first time you reconcile an account in QuickBooks Online. It will provide valuable information, as well as steps by step instructions on how to fix opening balance. 

If you have any other questions, feel free to reach back out  here.