Found your problem. You have some issues with your geo that I think started when you did the extrudes shown at 3:40 and 5:58 in the tutorial.
The edges shown in red should be single edges. The corner of the arches have 6 edges and the base of the lid has 4 edges.

Easy way to clean this up is to run a mesh cleanup. That will get rid of the faces on top of each other. The arches can be cleaned up by merging the vertices using a threshold of 0.01. The geo on the inside has the same problem.

You also have some rogue vertices on the side.