Why do I only get numbers while typing?

LetraTag label makers have a number lock feature.

Num Lock is turned off by default. When Num Lock mode is turned on, the Num Lock indicator is shown on the display (upper right corner) and the numbers 1 through 0 appear when you press the corresponding letter keys. To restore normal letter printing you’ll need to disable number lock by pressing the blue Num Lock button on the keyboard. The Num Lock indicator should disappear from the display.