Lightning App Launcher:
Knowledge tab is not visible in Lightning. I have checked all the permission, not sure why it's not appearing.
I went through most of the question on this topic but nothing seems to help.
The knowledge tab(s) (for classic knowledge) are not Available in lightning, you still have to switch to classic to have access them.
Unless you have enabled lightning knowledge, then you will be able to find it through the lightning app launcher once you have created the tab and linked it to the object (knowledge):
To do so, under setup, look for Tab, and create new custom object tab (custom object doesnt make much sense here, but thats the way it is(¯_(ツ)_/¯):
Also, you can check the following article Create Lightning Knowledge Home for adding a knowledge home page in lightning experience: