DIAGNOSIS The machine isn't detecting the tape cartridge. This is usually because the cartridge isn't pushed in all the way, or the little detection switch inside the compartment isn't being pressed.
FIX Turn the machine off. Open the door and take the tape cartridge out. Look at the bottom of the cartridge—there's a rectangular hole. Now look inside the compartment on the left side, you should see a small black plastic nub (the sensor switch). When you insert the cartridge correctly, that nub should fit into the hole. Re-insert the cartridge straight in, applying firm, even pressure until you hear and feel a solid *click*. Close the door. Turn it on. It should now recognize the tape.
IF THAT DIDN'T WORK The sensor switch inside might be stuck. With the machine off and the cartridge out, use a toothpick or plastic spudger to gently press the little black nub in and out a few times to make sure it moves freely. Also, check if the tape cartridge is definitely for the D30 series—some older or third-party cartridges might not have the correct detection hole.
HEADS UP When inserting, make sure the tape is feeding from the bottom of the spool, up and over the guide, and out the front of the cartridge. If the tape path is twisted or blocked, it can also prevent proper detection.