what does the error message 'Operation now in progress' mean?

Not directly related to the question of the OP, but if you are getting this error using the Laravel framework (or any other project using composer), chances are that the autoloader can't find one of the classes. For example this tends to happen if you move a class to a different directory without dumping the autoloader.

To fix this, try composer dumpautoload.

This occurs when there is an outstanding blocking operation.

In this context, the error implies that another process has a lock on the file, most likely due to the file being open and written to by whichever process holds the lock.