I have this weird effect after I apply shade smooth I am creating a glasses by following this tutorial
https://www.youtube.com/watch?v=9rGp_-zgjxQ
At 12:09, after I apply shade smooth on the frame and the lens. It has this weird effect.

I've been following closely to the tutorial. Is there any other way to solve the problem or I should just accept the flaw?
Did you select autosmooth check box?
It is right here, it should most probably solve your problem:

If it doesn't fix it here's is another possible solution:

Click on the arrow next to viewport overlay
Select 2nd option from the bottom called face orientation.
If all the faces are blue, you are good to go, if any face is red you might have to recalculate the normals.
to recalculate normals, go too edit mode, select all the face by pressing A, press Alt + N, From the list that opens select recalculate outside.