can't add actionscript to buttons

Hi, I am currently trying to make a game in AS3 with forward and back buttons. I have drawn my buttons and converted them to symbol (button), but when I select the button and press F9 to add in the actions, this happens meaning I cannot add the script in:

can anyone please tell me what I have done wrong to not show the normal space where I add my script into?

EDIT: OK, I have worked out how to code buttons I think, but now whenever I do I keep getting this error message