You probably have a tag with the same slug. You can’t have any taxonomies with duplicate slugs. Either delete that other taxonomy, rename/change its slug, or change/rename this category.
EDIT
Ok, it turns out you can do this, if you don’t mind programming the fix or directly executing the SQL or messing with PHPMyAdmin. What you need to do is add a new row in wp_term_taxonomy that uses the term with the slug you want to use as the term_id and the new taxonomy (category in this case) as the taxonomy. This is necessary because, if the slug already exists, WP creates a new term rather than using the same term with a different term_taxonomy relationship.
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