Confirm whether the paper roll is thermal paper:

Thermal paper can only be printed on one side. Use your fingernail or any other sharp object to scrape the paper. If any color appears on the paper, it is thermal paper. Install this side facing the print head.

Clean the print head using an alcohol cotton pad. Make sure there is no dirt or adhesive residue on the print head. Allow the print head to air dry and then try printing again.