"The lecture however does cover a lot of information, still doesn't explain the main subject."
You could say "The lecture, however, does cover a lot of information while still not explaining the main subject." if you're stuck on where however goes.
You could say "However, the lecture DOES cover a lot of information while still not explaining the main subject." or "The lecture DOES cover a lot of information; however it still doesn't explain the main subject" if you're stuck on using however and don't mind a semi-colon
I would probably say "While the lecture does contain a lot of information, it still doesn't explain the main subject."
Hope this helps