UDF FIELD FOR UNIT OF MEASURE CONVERSION MAINTENANCE

I have created a "description" field to hold the description of the unit of measure as the 4 letter unit is often pretty cryptic.  I had no problem creating it, but when I go to add the field to the panel (Dmain), I cannot add my UDF.  The add field is greyed out and will not allow me to add the field, though I have been able to add a placeholder description, which is now just sitting there by itself waiting for an entry screen.  In the beginning, I was going to require entry of the UDF, but without being able to add the field to the panel, it throw out an error, saying that the field was required.  My point of mentioning this is the panel recognizes that existence of the UDF, but there does not seem to be a way to force the Customizer to allow me to add it.

Any suggestions?