When you receive such error in live or test mode – it means that your website already has some plugin which uses Stripe library, and since our plugin uses the same Stripe library – there is a PHP conflict. 

To fix this – you need to edit  wp-content/plugins/stripe_payment_terminal/terminal/stripe/form.processing.php and comment out the line 169 which includes the Stripe.php library (just add two forward slashes like this //  in front of the line 169.