Ted here from Shopify. :-)
I was able to replicate the issue and it seems that the integration for the payment gateway 'Mercado Pago' is not working correctly. I'd like to raise this further with our Tech team if that's OK? To do so, could you email firstname.lastname@example.org? When you do, you should receive an automated email back with a ticket number assigned. If you let me know what that ticket number is, I'll take it out of the queue and raise it for you straight away.
Let me know how this goes!