First semi-difficult customer

Series of event:

Customer orders product for $900, I ship as I am FBM. I’m sort of new to amazon under a year of experience.

One week later customer says product doesn’t work. Okay whatever it happens. I message them back ask them to initiate return. No response from customer.

Day later Amazon says issue is reported from customer as “Unresolved”, but customer hasn’t responded to message or initiated a return.

Full quote from Amazon: “The buyer has expressed that their issue remains unresolved based on your last response. Please either continue the conversation with the buyer to reach a mutual resolution or select ‘No Response Needed’ to resolve the conversation despite the buyer’s feedback.”

What do I do here? Can I initiate a return from my side? I’ve never had to and don’t know how to. I’m fine with refunding them but I need to have the item returned to me.