1. If you get an error or "checkout with cart failed" or similar then take a screenshot, however your individual tablet handles doing that.
2. Contact Patreon so they know you're having problems. In the contact send the screenshot. If you can't attach it then upload it somewhere and put the link to it in the email for them. Tell them exactly what you were trying to do, and what sort of tablet you are using, and what operating system it is on (iOS and type, android and lollipop or whatever, linux type, whatever you're using.
Patreon help page: https://patreon.zendesk.com/hc/en-us
The "send note" link is right above the tweets. Patreon help crew is very helpful in my experience.
3. Contact Stripe in the off chance that the problem is on their end and not Patreon's site. You will need to tell them the same things that you should be telling Patreon in step two.
Stripe support: https://support.stripe.com/
Now that you have let web development know on both sides what the details of your problem are, whichever end the problem is on should be getting ticketed and checked into by their teams. Use similar steps if you have such failures on other sites too, just making sure to contact the appropriate people. Sometimes it's a simple thing, sometimes it's something that requires a bit more time. Letting a site's development team know something is going on that it odd is very helpful to them, as there are times they won't know something has decided to go "nope, not today. Stars..."