An error 1120 is ‘Access of undefined property’ which can means two things. Firstly, you are trying to access an undefined variable, and secondly you have not given a correct instance name to a movie clip or button.
To solve this problem you need to make sure the instance name field is not empty and contains a valid instance name with correct spelling.
You also need to make sure you make the correct reference to the instance name as shown below.
This should solve an 1120 AS3 error.