Check the followings.

• You tried to enter a 3rd line.
  In this case, you see two return marks "" in the display.
  Clear either of them, and press the [Print] key.
   
• You tried to print or preview two lines of text while 3.5 mm / 0.13″ or 6 mm / 0.23″ tape is installed.
  Install larger tape size or clear the return mark "".