can not add signature

 There are a couple of things I noticed in your video that seemed odd:

Thing One: You seem to have at least two versions of Adobe Acrobat on your Mac. There was an icon on your desktop for Acrobat, and there was a second to the right end of your applications that showed another version of Acrobat was running. What version(s) and types (Standard? Pro? Premium? Reader?) do you have installed on your Mac.

Thing Two: To add your signature, first you have to have a signature field where you can put it in your document. Then you can add a signature to your PDF document. For more information on how to create signature fields in Acrobat and then fill them in, let me refer you to the following link:

https://helpx.adobe.com/acrobat/desktop/e-sign-documents/fill-sign-documents/add-esign.html?x-product=Helpx%2F1.0.0&x-product-location=Search%3AForums%3Alink%2F3.8.0

You may want to check out some of the follow-on links on that help page. It’ll help you develop a well-rounded background in how signatures work in Acrobat, and how you can best apply them.

Hope this helps, 

Randy