The file is not checked out. You must first check out this document before making changes.

The file is not checked out.  You must first check out this document before making changes.
When we see exception like 

Please make some steps below to prevent the exception with MOSS 2007 (Microsoft Office SharePoint Server 2007)
1. Goto "Modify Pages Library Settings"

2. Click on "Versioning settings"

3. Choose "No" for "Require documents to be checked out before they can be edited?"

God bless you!