Issues with creating solid via surfaces

I have a set of surfaces that form the shell of a solid I want to create. However, I've been having trouble getting it to make the solid. All surfaces are knit together, and when I created the last surface via a fill, it gave me the "create solid" option after I chose to merge. However, upon completion of the fill, the surfaces were not converted to a solid. Additionally, trying to thicken the surfaces does not give the option to create a solid, which would mean that somewhere is not "airtight," but I'm at a loss to where that is. I did use the geometry analysis and found things like four knife edges are present in the model. Would those knife edges be causing this?

I've attached a link to the part. Thanks in advance