You need an SSL certificate for the domain that directs to your Pattern site. You should take a look at this
Etsy is supposed to take care of everything for Pattern site owners.
This has been on ongoing problem with Etsy's Pattern site for years. Since Pattern has no support nor updates done to it, the issue is likely to be ongoing. There were some teams dedicated to supporting peer site users. I'm not sure if these are still active? It may help to do a search via Etsy teams. Or, this may be a good time to start another website with a company who supports their product? While I love my Etsy shop, I found Pattern was extremely lacking.