What version of PCM are you using?
Has attachment ever worked successfully before without the error?
Does the file actually attach, regardless of the error?
What method of attachment are you using? File Server or Content Repository??
For testing purposes, try to use google chrome or firefox and try to attach the files. If succeeded, then the problem that you are facing might be from Internet Explorer.
Edited by: user8981205 on Feb 1, 2013 2:22 AM
I had that error occur in CM14 when we had 'Version Documents on save' was checked but no Content Management setup.
under Admin, Content Management.
It will be important to know which CM version you are using, and also which repository (UCM/SharePoint/FileServer) method. As Laban_user points out, if you are on File Server, and not have an active Repository, checking the document Versioning will give you a repository error.
If this is not the case, let us know which PCM version, etc. It is also possible that you have a configuration error in your command line server admin setup that has the attachment path incorrectly defined.