The Unity error message 'cannot build player while' usually surfaces when there is an ongoing process in the Unity editor that hasn't been finished or terminated properly. These processes could range from scripts that are still running, asset import operations not yet completed, or perhaps scene loading that is currently underway.
Sometimes, this error can also be a result of inconsistent or corrupted project files. If the Unity editor isn't able to read or access necessary files due to corruption, it wouldn't be able to build the player effectively, leading to this error.
To resolve this issue, you'll want to start with the simplest solution: stopping any ongoing processes within your Unity editor. Try closing and reopening Unity to ensure all running scripts or operations are stopped.
If that doesn't work, try to isolate problematic scripts by disabling them one at a time and trying the build again until you find the one causing the issue. Be sure to look for scripts that may be stuck in infinite loops or long-running operations.
For assets that might be in the middle of importing, check your 'Project' window to see if there are any assets currently being imported. If there are, allow them to finish, or cancel and retry the import operation.
Lastly, if corruption seems to be the issue, consider creating a fresh project and slowly migrating your assets and scripts to the new project. Always make sure to back up your projects before doing so to avoid any data loss.