let me know how to resolve
This is probably going to be a “best of luck, let us know how you get on”. That’s not to say that folks here (employees and customers) won’t help you; more that, as your question stands, this is too broad to answer.
Fly is basically like lots of Docker containers that are quick and cheap to create and destroy. What one does with them is up to the engineer. If you have a Shopify widget that would run on Node/Linux then chances are you can get it working on Fly. Does this thing come with a Dockerfile?
Perhaps start with the docs we have on the subject? Shopify · Fly Docs