The file you pasted got the formatting wrong when put on itch. Can you put the contents of the file on pastebin.com instead?
Deleted account1 year ago
Deleted post
Cool. Looks like there is something weird with the geometry on the upper half of the missile. Look here: https://ibb.co/JFkNQyb
I believe it is due to the top vertices being collapsed into a single point, that causing it. That causes both the renderer to mess up the normals for the backface culling and then the exporter probably chokes on the top surface of the hexagon being reduced to a single point.
To fix it, redo the top half of the missile and remove the top face before merging the vertices. Then set all sides as "dbl-sided" and thy will render correctly.
Good luck!