There is an attribute with a drop-down list and the value for this attribute is already selected. If you want to delete this value (change it to be blank) but it only lets you to choose some other value. How to change value to blank?
This means that there is 'Limit to List' checkbox selected on this attribute. To change this option go to Attribute Properties > Value tab and uncheck the 'Limit to list' option.
If you need to limit to list then the list will need to have a null entry in it. That will allow blank to be a valid option.