iOS can’t make shipping build

iOS developers please help me! I’m trying to get a build working on testflight, and having trouble with distribution certificates. On attempt to package, I get the error: "Signing key not found. The app could not be digitally signed, because the signing key is not configured. I created the certificate in .p12 format via xcode, and then downloaded the provision off of the Apple Developer website. I am hitting package in shipping mode. This is a BP only project. Thanks in advance.

Are you building this on a mac or pc?