Thanks Alin,
We installed the 5.1.18 version and it is the same with the Stripe plugin enabled.
We did an additional test though to make sure the problem is in the plugin and not Guru.
We enabled PayPal and Stripe this time.
When we try to apply the discount code when the PayPal option is selected in the dropdown list, it works. The amount for the course is decreased.
When we try to apply the discount code when the Stripe option is selected in the dropdown list, it doesn't work. The amount for the course is still the same and the card or last name information is asked to be entered.
We have also added some debug coming from Chrome in the console. The Sripe plugin is 1.0.4.