DIAGNOSIS The tape end sensor is not detecting the reflective mark on the tape backing. This is usually due to incorrect tape loading or a dirty sensor.
FIX First, verify correct loading. The tape must feed under the silver guide roller, not over it. The glossy, thermal-sensitive side of the tape must face down toward the print head. Second, clean the optical sensor. Power off. Remove the tape. Locate the small clear plastic window inside the compartment, near the feed path. Use a dry cotton swab to gently wipe it. Reinsert the tape, ensuring it passes directly over this sensor window.
IF THAT DIDN'T WORK The tape cartridge may be incompatible. The P21 requires tape with a specific black rectangular "start" mark on the backing. Some third-party tapes lack this mark or have a faint print. Test with an official Nelko cartridge.
HEADS UP Do not use clear or white tape that lacks the black optical mark. The sensor cannot detect it. The "out of tape" warning will persist. The sensor only reads contrast between the backing and the mark.