The problem is that the folder is already under version control.

Here‘s how I fix this type of problem when it comes up:

1. In your local shell, make a copy of the .settings folder
2. Bring up the "SVN Repositories" view in Eclipse
3. Navigate to within the project on the server (e.g. /trunk/cboon/Project1/)
4. Right-click the .settings directory there and delete it
5. Now locally, update the project. If it feels that .settings is in conflict, accept the incoming change.
6. Copy your local back-up of .settings back into place and refresh. It should now have a ‘?‘ badge, indicating that it‘s not under control
7. Now you can svn:ignore it

from:http://stackoverflow.com/questions/16646942/unable-to-add-file-or-folder-to-svnignore

add to svn ignore disabled