I can only assume you are using version 7.5.6
If you are using attachments not a gallery forum.
Goto
Control Panel » Feature Settings
Double check the path
Use the "click to test directory" link.
It should pass and create a file called test.file in the folder with a current time stamp.
Also I would check
Control Panel » Permission Checks
See if anything fails there.
Like tmp,cache,etc,etc.