Microsoft Power Point saving error

Hi,
I am preparing my presentation using Power Point but when I tried to save the document and error shows up “The file cannot be saved right now because the file is loaded by another user at the moment”. I am using Mac Operating System and a single user so how can I demolish this error? Help me find a solution.

The file cannot be saved right now because the file is loaded by another user at the moment. You can wait a few moments and try the save again, or you can cancel this save and resave using a different file name.
