This is kind of "intermediate" fix.

Go into your cpanel

Go into phpmadmin

Click on the database for your website from the drop down list (if you only have one website you may not need to do this)

Click on the table labeled wp_terms

Click on the name heading (at the top of the column) to sort the column

Look for the link name/word that you're trying to add

Click on Delete to delete that line. 

as always if you haven't backed up you database, it is recommended that you backup on a regular basis.