I signed up for Bill Pay Online. I'm trying to pay a vendor through a bank transfer, but the option is greyed out.

The bank transfer using Online Bill Pay is available in QuickBooks Online Plus and Advanced, asmith1.

It could be that you're using the Simple Start version of QuickBooks Online. If yes, you'll want to upgrade your subscription to use this feature.

Check this article for your reference about Online Bill Pay.

If there’s anything else that I can help you with, click on the Reply button and post your comment.