If you've never started a GPS activity or if the battery on the watch fully discharges, you may see a message similar to the one listed in the below image. This requirement will allow the watch to confirm the the current date and time.

deezer GPS message

To confirm this requirement:

Step outside for an open view of the sky
Access the activity list on the watch and choose an outdoor activity type such as running
Wait a few moment for the watch to acquire a GPS signal

You will know when the watch has acquired a signal by the messaging on the watch. Once this is complete you can access and use the Deezer Music app.