DIAGNOSIS The app's template size does not match the physical tape cartridge installed. The P21 detects tape width via cartridge pins.
FIX Check two things. First, confirm the tape width. Remove the cartridge. The width (e.g., 12mm) is printed on the side. Second, in the Nelko app, create a new label. Before designing, you must select the correct tape size from the settings icon. Choose the exact width that matches your cartridge. The error will clear. You must set this for each new label project if you change tapes.
IF THAT DIDN'T WORK The cartridge may not be fully seated or its detection pins are dirty. Remove the cartridge. Use compressed air to blow into the empty cartridge bay. Reinsert the cartridge until you hear a definitive click. Restart the app.
HEADS UP The "Multiple Templates" feature only works with supported widths (6mm, 9mm, 12mm). Using a 24mm tape, for example, will always trigger a mismatch error. The printer firmware does not support all template sizes on all physical tapes.