Welcome to the forum!
I'm not familiar with tinkercad. But if it's a CAD program, I would assume it has the ability to replace the fill?
But that said, I looked at your file. You have 20 something objects stacked all on top of each other. I'm not sure exactly how that might have happened, but looking at the 3d image you attached, I assume it was some kind of attempt to convert that to vector? So possibly the cad program was confused by the many stacked up objects.
Parts of the object on top looks like you might have tried to trace it with the Pen tool. But it appears it had a shape applied to it, making the paths pointed on one end and wider on the other end. That could have confused the cad program as well. To use the Pen tool in default mode make sure it's in Regular Bezier mode, and that the Shape dropdown menu is set for None. You can find those on the control bar (the first bar of icons above the horizontal ruler, which changes with each tool which is enabled)
I would suggest to focus on having just one object.....or at least one version of each object. In some places, you have just the outline path, and other places only the fill path, and yet other places seems to be some combination of both. So I'm not sure exactly which piece you need, in the end.
Also, it looks like you might have used the Paint Bucket tool multiple times, perhaps trying to fill up the gaps that happen, if you don't have the tool configured properly. On the control bar look for the Grow/Shrink setting, and set it for something like 1 px. Then using trial and error with Undo, find the best setting. That way, you can fill the area in one click.
If you could explain a little bit about your project, we might be able to suggest a better approach, depending on your end goal. I mean, if there's some reason for all those separate versions, possibly Inkscape can't do what you want, or possibly there's a better way to do it?