What means this error?
The forward declaration
typedef struct _Dictionary_t *Dictionary;
doesn't give a clue as to what members struct _Dictionary_t might have.
You cannot use main_dict->size unless the definition of struct is visible at that point in the code. Since the definition of the struct is in dictionary.c, its definition is not visible to anything in main.c.
How I can fix it?
Solution 1
Move the definition of the struct to the .h file.
Solution 2
Provide functions that allow you to get and set the members of the struct.
In dictionary.h:
void setDictionarySize(Dictionary dict, int size);
int getDictionarySize((Dictionary dict);
Implement them in dictioary.c.
Use the function in main.c. Replace
a = main_dict->size;
by
a = getDictionarySize(main_dict);
