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Is this the form you are working on https://www.jotform.com/61477497499175?
I have inspected that form and I saw that you are passing calculation to payment field. Considering that calculation will be zero if non of the options are selected, zero will also be passed to payment field. Unfortunately it is not possible to leave it blank.
Possible workaround is to set condition that will hide payment field if calculation is equal to empty.
Here is my demo form: https://www.jotformpro.com/form/61785726944975
Hope this will help. Let us know if you need further assistance.