Stripe.js token was not passed!
This seems to be a long-standing issue with the Stripe processor, and from what I can tell (and from what I have experienced), in some cases this extension doesn't play well with other payment processors (including the "pay later" option). 

Try disabling Paypal (and Pay Later, if applicable) and then test Stripe again.

See also:
https://github.com/drastik/com.drastikbydesign.stripe/issues/76