Can't add action to button

The  form is probably set up to be an Adobe Sign compatible form, which happens when you select "This document requires signatures" when you initially select: Tools > Prepare Form

You can change it to a regular form by selecting: More > Revert to Acrobat Form

while you're in form editing mode.

What do you see under the field's Properties, then?

[Question moved to the Acrobat PDF Forms forum]