Ted here from Shopify, thanks for reaching out to us.
In regards to the unavailable message, some admins were not loading correctly for a few moments for some merchants. That issue should be resolved but if you're still seeing the message please let me know.
For your gateway query, are you adding the test card numbers provided in this guide? If so, which card have you added to the test? Would you be able to share a screenshot of the test error with me?