If you have Spotify set as your default music service, then your Echo device might not let you set music as your alarm. The trick is that using music for an alarm has to come from Amazon Music. So just add “on Amazon Music” at the end of your request. Like this:
“Alexa, set an alarm for 6:30 AM to TobyMac on Amazon Music.”
This works on Echo, Echo Spot, and Echo Dot. Enjoy.