There can be several reasons behind the 'Unity cannot open project' error:
Incompatible Unity Versions: You could be trying to open a project created with a newer version of Unity using an older Unity version. Unity does not guarantee backward compatibility.
Corrupted Project Files: Your project files might have been corrupted or accidentally modified. This could happen due to various reasons like unexpected system shutdown, disk errors, or even manual modification of files.
Missing Assets or Broken References: If some assets used in your project were deleted or moved from their original location, Unity will fail to load those assets which could cause the project opening process to fail.
Incorrect Script Compilation: If there are scripts in your project that have compilation errors, Unity might fail to open the project properly.
Solution - Here's How To Resolve It
Based on the causes mentioned above, here are the corresponding solutions:
Check Unity Version: Make sure you are using the same Unity version that was used to create the project. You can see the Unity version in the 'ProjectVersion.txt' file located in the 'ProjectSettings' folder of the project.
Resolve Corrupted Files: If a specific file is causing the problem and you know which one it is, try replacing it from a backup or creating a new one. If you're unsure, consider creating a fresh project and gradually migrating assets and settings from the problematic project.
Ensure Asset Integrity: Make sure all the required assets are available and in their correct paths. Use the Unity's 'Reimport All' option (Assets -> Reimport All) if necessary.
Fix Scripting Errors: Check the Unity console for any scripting errors and fix them. If you can't open Unity at all, check your scripts in an external code editor for syntax errors or other issues.