Stripe plugin fails in checkout even in testing mode

I know there are some topics related to Stripe Plugin problems with Stripe gateway. but I am not even able to use the test keys and simulate payment. The Debug mode is on, the moment I click on the Stripe Payment button it redirect me to the listing page with error.
Please have a look at the snapshot.
The browser console log error is : Failed to load resource: the server responded with a status of 500 ()
and I receive an email saying : The status of your order No. B2G9 has been changed to cancelled.
and Stripe dashboard doesnt have any customers details, or anything else.

Also, I get this error message when the payment fails, but the debug mode is off:
Error charge : No such customer: cus_BVB6yF0WxFBUks

any idea what should I do? PLEASE PLEASE give me some ideas.