This usually means the tape isn't seated correctly or it's the wrong type.
First, open the housing and check the tape installation. The round black piece on the tape spool must be on the right side. Pull and turn a piece of label tape out, then close the housing firmly.
If it's still not recognized, you're likely using a non-SUPVAN label tape. The printer only works reliably with original manufacturer products. Switch to a SUPVAN tape.