When I pay a bill I am usually able to give my payment a reference number - but that box is now greyed out and it has the words "To Print" I can't change it or edit it

Thank you for taking the time to post here in the Community, @acengineering. I'll make sure you'll get to add a reference number when you pay a bill.

You can enter a reference number if you're paying bills with a bank account and uncheck the Print later box to eliminate the To print in the Starting check no. field. You're unable to see the Print later if you're using a credit card account when paying a bill.

Here's how:

1. Click on + New and select Pay bills.
2. From the Payment account use a bank account type of payment.
3. Then uncheck the Print later box to eliminate the To print.
   
   
   

I've also added this article for your future help: Enter bills and record bill payments in QuickBooks Online.
 

In addition, here's a link that explains all you can do with the vendor's functionality.

When you pay a bill in QuickBooks Online, tap the Reply button if you have any clarification or additional information. I'll be here for you. Enjoy your day!