A locker room is where people change and get ready among others, so it is read as vulnerability, comparison and preparing for a challenge. A locker that will not open stands for something you cannot reach in time.
Has this come up in your dreams before? What was going on in your life then?
My Cosmic Code has a private dream journal. Write a dream down and the app picks out the symbols in it, shows what dream books say about them and, over time, which words recur across your entries. The journal is stored on your device only.