Wont let me save my document

Every time I try to save my document I get this:

The document could not be saved. The file may be read-only, or another user may have it open. Please save the document with a different name or in a different folder.

And when I click ok, I try to save as into another file but I get this:

The document could not be saved. A file error has occurred.

I have a mac. Someone, please help!