For the PT-D600 and PT-D600VP, Brother specifies TZe cassettes in 3.5, 6, 9, 12, 18 and 24 mm widths. Its loading instructions explicitly exclude older TZ and HG cassettes for these models.
Open the tape-compartment cover and check the cassette reference before seating it. Do not force the cover shut over a cassette that is not sitting flat.
Check the release lever
Brother says the white release lever should be up when the cover is open. It moves as the cover opens and closes; do not push it down manually.
If the lever is down with the cover open, follow Brother’s instruction to pull it up with your fingers before inserting the cassette.
Seat the cassette and close the cover
Align the printer’s plastic posts with the slots in the cassette. The posts allow the cassette to be inserted in one orientation.
Press the cassette down firmly. Keep the tape and ink ribbon clear of the printhead so that neither becomes caught. Then close the cover until it is fully seated and level with the lower housing.
If it still will not seat
Recheck TZe family, width and alignment. A cassette that cannot be installed or a cover that will not close needs attention; repeated force can damage the feed path.
These are the PT-D600/PT-D600VP instructions. For another P-touch model, use its own loading diagram and width list, because cover and lever details can differ.