In the model attached I am trying to print these surfaces (the cones) at a width of 420 mm. From what i have read to 3d print the surfaces should al be able to join so there are no “leaks”. I am unable to join the surfaces together. Would someone be able to help me? Thank you!!
your model consists of mostly open surfaces which are joined. some can not join because you would create non manifold surfaces as for instance in this area where the edges of a surface connect onto a surface and not onto further edges.
for it to print at all you would have to give each wall a thickness (which will also solve your non manifold problem) and join it together in a meaningful way that you get a closed polysurface.
In your file some edges are visually pretty close but not close enough to join, others are more clearly in need of trimming. I think you want something like this, all jouned up, as a starting point to thicken