Figure it out. For some reason, I had to convert the form to an AcroForm. I don't now what this means as the PDF was created in Acrobat; however once I selected this "conversion", the digital signature option appeared again.
It means you selected the "This document requires signatures" when you first selected "Tools > Prepare Form". What this really should say is "This document will be used with the Adobe Sign e-signature service". When selected, you're limited in the types of form fields that you can create to those that are supported by Adobe Sign, which excludes buttons, digital signature fields, and 2D bar code fields.
Thank you, you are 100% correct. To clarify my issue, I did select Tools >Prepare Form upon PDF creation, however, it defaults to an "Adobe Sign Form" which does not allow you to add a digital signature field.
So while in the Prepare Form area, I had to select More > Convert to Adobe Acrobat Sign Form, from the Right Panel Menu (above the "Fields" area). If you are not in this format the digital signature option will not appear in the toolbar.
Defaulting to the Adobe Sign to push e-services in not a good idea Adobe.