One of the fields on my form suddenly can not be selected...(Local Dues field )

The last time the form was used with all enterable fields was Dec 8.  Since then the field can not be selected.  In looking at my history, I haven't made any changes after Dec 8 until today as I attempted to see what's wrong with the field.  It does appear that the features are not operating as they were when I created the form.   

This can be done through the following settings :

Step 1

Step 2

Step 3

Hope this helps.