Functions and variables have a visibility, an "Access Modifier":
public - Accessible from this contract, inherited contracts and externally
private - Accessible only from this contract
internal - Accessible only from this contract and contracts inheriting from it
external - Cannot be accessed internally, only externally. Recommended to reduce gas. Access internally with this.f.
You should get used to assigning access modifiers to your functions, atleast, to get that error to go away.