I am on the last module of the Build a Discount Calculator trail. I have verified the first 4 steps and received the 50 points (everything was configured correctly). Now, when I try to verify my last step I am getting the following error:

Challenge Not yet complete... here's what's wrong: 
The 'Calculate Discount' custom button doesn't exist or isn't properly configured. Check the instructions.

I've verfied the creation of the button, it's on the correct page layout and I've tested the button- it gives me the correct discount percentage and discounted amount.

Any other ideas on what I should check? I'm at a loss.