The specified grid to use as the mask could not be opened. This may be due to incorrectly specifying the path to the grid or not having permission to access that location. If the path and permissions are okay, the next possible source of the problem may be missing required component files in the grid's folder. If all the required components are present, it is also possible that the files that contain the binary raster data (the w00n and w00nx files) are internally corrupted.
Check that the grid exists and you have correctly identified it. You may also need to check that you have the correct permission to that location. If this has been done and the problem continues, you should then determine if the grid is valid. First try displaying it. If you cannot display it, check to see that all the required component files are present in the grid's folder. If there is no sta.adf file in the grid directory, try creating one with the Calculate Statistics tool. If its data files seem to have been corrupted, you may need to re-create the grid. Hopefully you have a backup copy that can be used or can run the process that created it again.