How do I fix the two black lines on my mesh? I pressed F to connect an edge between vertices in order to create a straight line where I could mark a crease. But The new lines created by pressing F are behaving weirdly. Here's a blend file https://drive.google.com/file/d/1l4dyZFzKD4OYRlZNAnEdIrgnLw0WsxTN/view
You have edges going across faces.

Remove the edges that go across these faces by selecting them, pressing x and selecting delete edges.

Next select the vertices that were left behind and press j key to join these vertices

The end result is quite good.