Thanks for your reply, Anubha. There are no updates to run in my Adobe Acrobat DC, and the tool is still not there. I heard that the tool is not available for non-subscribers.
after a very long chat with Adobe Support and trying all their suggested solutions, which none of them worked.. I found the answer to this problem.. big thanks to alisterblack in his reply to Re: Acrobat Pro DC - subscription is missing tools
Acrobat just disabled all three tools related to signature if your Application Language is NOT (English only). mine was English with Arabic support, and did not work until I changed it back to English.
I really do not understand why this is not documented properly by Adobe. And why you need to withdraw your language requirements in order to sign a document??