I think I can see where your problem has happened. I have built a few of these Stewmac kits.
The kit is designed that the neck set angle is at zero degrees. That is, when the bare neck (without the fingerboard) is placed against the body face down on a flat surface, the top of the ukulele (the soundboard) is dead flat and the neck face dead flat in relation to eachother. The heel is already pre-shaped to match the top of the body.
Like this (this one is a Stewmac kit, but with added binding and purfling)
You see... the body and the neck are face down on a flat surface, and this is the "zero angle" of the neck set.
Now, that zero degree neck set angle can only be achieved if the top end of the body is built vertically. It looks from the photo that yours isn't. Therefore, when you fitted the neck, it is not in the same plane as the body
top.
This exaggerated drawing shows what is happening on yours.
In other words it is a compound error... if the neck block wasn't dead vertical, then when you glued the sides to it, they would not be vertical either. Hence the top end of the uke isn't vertical (shown by the red line in my drawing). Then when the neck was fitted, its face isn't vertical either. That is the result of your high action.
See the photo by "Rod True" above... his red line shows the problem too.
A little "bodge" was needed when you set the neck in place to allow for that slight angle misalignment. This still could be done if you can get the neck off. Wood needs removing from the business face of the heel (where it meets the body). i.e. at the narrow end of the heel face. That would bring the neck (fingerboard surface) back into alignment with the top
face of the body.
Rod