The Zebra Barcode Printers support Thermal Transfer (use of ribbon) or Direct Thermal (no ribbon required) printing.

Direct Thermal labels: Uses a heat-sensitive coating that will darken when it is heated to produce an image.
Thermal Transfer labels: Do not have a heat-sensitive coating. Hence, you would need a ribbon for image transfer.

To check if you are using the correct labels for the Direct Thermal application, perform a simple scratch test. 
Watch the video in the link below to see how scratching the label with your fingernail should produce a mark:

https://supportcommunity.zebra.com/s/article/Testing-labels-for-Direct-Thermal-Printing-Applications?language=en_US

If the scratch test does not leave any marks, consult your label supplier.