You only need to click on the signature field to add a signature, the feature under "Fill&Sign" is for the case where the document does not have a signature field. You cannot use a certificate based signature when the document has a signature field.
In this document, from what I can see, the signature field is not enabled, I assume that once all the required information is added, that field gets enabled and you should be able to use it. If that's not the case, get in touch with the author or distributor of the form to find out what the exact requirements are for enabling the signature field.
Karl Heinz Kremer
PDF Acrobatics Without a Net
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