I'm creating a fillable PDF. I need to figure the totals of some of the cash amounts other fields. I have a text field for that and under properties I have in the calculate section,I have Value is the sum + and when I try to pick the fields to add, i

I'm creating a fillable PDF. I need to figure the totals of some of the cash amounts in other fields. I have a text field for that purpose and under properties in the calculate section, I have Value is the sum + and when I try to pick the fields to add, it doesn't let me check them off. How can I get them to get checked? Thanks for any help.