Why is the Place Order button greyed out for one customer's address?

When the button turns grey only after the address is entered, it is almost always a delivery problem. Look for a red notice above the button:

• a product named as "not available for delivery to this location", from your Delivery Restrictions rules
• your own not-serviceable message from Delivery Restrictions, when you don't deliver to that pincode at all

The customer needs a different address or must remove that product.

The button is also disabled when:

• the Terms and Privacy Policy box isn't ticked
• an item is out of stock (it reads Remove out-of-stock items)
• your store is in demo mode (Ordering disabled (demo))