A door, gate, portal, etc. can represent a passage or transition from one area of space or time to another. It can represent:
A door inside a house might represent transitioning between areas of your life (work life, family, etc.), and a house's front door might represent the boundary between your personal life and your public life.
Not being able to open a door can mean you're feeling stuck, unempowered, or believe you're being held back by others (people, God, the universe, etc.) somehow in your life. Make sure you're taking responsibility for your life instead of blaming others.
A door that offers access, such as a door on a refrigerator or a safe, could represent the idea of access or permission to access something (an area, a person, certain information, money, other resources, etc.) in your real life.