Where are you saving the file while working on it? Are you using a cr acked version of SketchUp 2021? Did you have a power failure while working on the file? The file is missing some important data. Did you try renaming the .skb file to .skp and opening it?
Well, first of all, using cr acked software is illegal. It amounts to theft at a minimum. Coming to a forum owned by the company from which you’ve stolen the software isn’t smart. It’s like stealing a car and then going back to the owner for help when it won’t start.
Advice: Get a legal subscription first. See this link. Then install the current version of SketchUp for Desktop. As a student you can get a very inexpensive EDU license to use. After that, start over on your project and make sure you are saving frquently.
Thank you for the advice, DaveR. I appreciate your time and the clarification on the community guidelines.