When you click Submit Evidence, each attached file is first uploaded to Stripe and then the evidence is submitted. If Stripe rejects a file (unsupported type, too large, or the dispute is no longer accepting evidence) the window shows "Stripe: <reason>" in red and nothing is submitted, so you can fix the problem and try again. Use PDF, JPG or PNG files of reasonable size, make sure the dispute still shows Needs Response with time left, and check that the kitchen location's Stripe account is connected ("Invalid Keys" means it is not). Because evidence can only be submitted once, do not retry until the error is resolved.