Please tell how to divide Links into Categories?

In the table LINKS there is a field named "category", but when I try to fill this field with some words, an error occur. What's wrong?

What kind of information is this field for? May be it is related with some other table in the database?

When you add a new link, select "Other" from the Category drop-down menu. Then enter your new category after the "If other, specify".

bye, Dirk