Where do you see "Create Custom Tool" ?
When you open Acrobat DC click on the Tools menu option at the top left
Scroll down until you see Customize
Open the "Create Custom Tool"
Expand "Prepare Forms" in the list displayed
Select "Digital Signature" (second from the bottom)
Add it to a custom list, or add it to show in the Toolbar
I strongly recommend reviewing the entire list which displays when you open the "Create Custom Tool"
Click save and name your custom tool
You can now access the "Digital Signature" tool
It baffles me that "Digital Signature" isn't part of the default "Prepare Forms" toolbar
The "Digital Signature" is part of the default "Prepare Form" toolbar. But not for a Adobe Sign Form.