My family lives on the fourth floor and gave me a small terrace. I want to seal it up. The property management said that it needs the consent from upstairs.

Since it is a gift for your family, you should have the right to decide whether it is safe or not. The reason why you don't agree upstairs may be that you are afraid that the installation will cause security risks to their home. You can consider this problem when installing. If there is no security risk upstairs, he should not interfere with you. For reference only. ...