You have some conflicting diagnostic processes here.
I’m suspecting the SSD’s format was not setup correctly. You need to format a raw drive with GUID either with HFS+ (Journaled file system) or APFS (if you are using High Sierra or newer). So with an older model you like this one we can mess up without realizing using a newer Mac to prep the drive! Here we need to use HFS+ as the system has no means to support APFS.