Can’t sort Posts in wordpress Dashbaord

The only time I’ve seen this happen is with a theme using JavaScript that is conflicting with the JavaScript in the WordPress administration area. If this is what is happening to you then its a poorly-coded theme and you should change to a different theme.

If this is a premium theme you should contact the theme developer (if you know who that is). Since you have labelled this as a wordpress.com issue you should also check the wordpress.com support options for help if you are set on continuing to use that theme.