Could anyone tell me what is wrong with my code? Im a newbie and I have no idea what is wrong.

Advice: follow tutorials to learn the basics of python.
You probably got error messages

def cube(number):
    return number**3
    
def by_three(number):
    if number%3==0:
        # you forgot to tell what you cube()
        cube()
        # here, cube is a variable that does not exist
        # the '.' at the end does not exist
        return cube.
    else:
        return False

There is a tool that allow you to see what your code is doing, its name is debugger. It is also a great learning tool because it show you reality and you can see which expectation match reality.
When you don't understand what your code is doing or why it does what it does, the answer is debugger.
Use the debugger to see what your code is doing. Just set a breakpoint and see your code performing, the debugger allow you to execute lines 1 by 1 and to inspect variables as it execute.

Debugger - Wikipedia, the free encyclopedia[^]
27.3. pdb — The Python Debugger — Python 3.6.1 documentation[^]
Debugging in Python | Python Conquers The Universe[^]
pdb – Interactive Debugger - Python Module of the Week[^]
The debugger is here to show you what your code is doing and your task is to compare with what it should do.
There is no magic in the debugger, it don't find bugs, it just help you to. When the code don't do what is expected, you are close to a bug.