Charlie wants a signed agreement from Lucy before he kicks it. And he will need a Pen for Charlie to sign, give him the pen you found on Linus’ desk when he asks for it.
How to get the Pen
You will need to get first a Sucker candy, when you got it. Head back inside the Van Pelt house, you will find Linus writing the Great Pumpkin, give him the lemon sucker and he’ll give you his Party Invitation, he will also left a Pen in his desk, take it to your backpack.