Thank you so much!
So these issues tend stem from surface not matching up at edges correctly?
How did you spot that by the way?
Not a problem.
Yes, if you are unable to create a solid when knitting the surfaces together it suggest that there is either a gap which is too large or that the face edges do not line up.
If you try selecting a face and rotating the model round you can tend to see the issue. It is relatively simple for this model as it only has a few faces. It can be a little harder when its a larger model.
My advice when using Surfaces is to not try and connect the edges, when you are going to make an enclosed shape.
Use Planes and 2D sketches where you can and not 3D sketch (along with fully defining your sketches, even if you are just practicing. Practice make permanent)
Knit as you go. Makes it easier instead of trying to find issues at the end
Mutual Trim ensures Surface join
The Delete & Fill feature takes an enclosed Surface entities and turns it into a Solid