I am unable to complete this module due to 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.

https://trailhead.salesforce.com/en/projects/flow_calculate/steps/flow_calculate_implement

I tried renaming the button API name to 'Calculate_Discount_button' as suggested in other threads. But this is not working in my DE. 

Any suggestions?
 

I am unable to complete this module due to 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.

https://trailhead.salesforce.com/en/projects/flow_calculate/steps/flow_calculate_implement

I tried renaming the button API name to 'Calculate_Discount_button' as suggested in other threads. But this is not working in my DE. 

Any suggestions?