For Testbook Website:
You can ask your doubts in the Doubts section of your Super Coaching. We are also enclosing the screenshots of the same for your reference.
For Testbook App:
Please check below Supercoaching tab > Just below Dashboard section.
To Ask doubts in a Course under your SuperCoaching:
For this, you need to select your course under SuperCoaching in that you want to ask a doubt.
Doubts & Discussions for courses are available under your Supercoaching > Your Course > Doubts & Discussions Tab.
Web Screenshot
For Testbook App:
Just open a course and go to the right of Study plan tab > Doubts > Ask your Doubts box.
Hope it helps, Happy Learning!