Cannot convert closed surfaces to 3d solid.

[jvernonXSGXL]

I cannot figure out how to convert this object to a solid. The surfaces are as closed as I can make them unless there is a trick I'm unfamiliar with. I can see that there are small gaps when I zoom in, but it appears to be a matter of parameters within the generative algorithm. I've attached the file for your reference.

[Patchy]

Follow how to do it here:

https://www.youtube.com/watch?v=Gz82S8RQMS4&t=711s

[jvernonXSGXL]

That's the tutorial I was doing when I started having this issue. I've done every step exactly the way the video shows multiple times. Everything goes smoothly until I reach the step of turning it into a 3D solid. It gives me an error saying no watertight volume detected. I'm at a loss. I took out the two holes at the ends to simplify the overall design but I can't get over this hump.