For anyone else that runs into this issue. 

It wasn't clear in documentation and this was initially set when my Una Cloud server was provisioned. Another Una member told me to delete the key/secret for HTTPS -- wondering if that was even correct now -- so the Key/Secret in Settings > System > General no longer matched what was in my Key/Secret within My Sites page on Una.

Your Key/Secret under My Sites on Una.io should be entered under Settings > System > General in Studio. If you delete the key/secret and create a new one, then you will need to update it in Studio. This is required for authentication to allow you access to the Apps Marketplace.