From a very quick look I can see by using the sub-select tool that your shape is made up of 5 separate paths that are not joined together
The reason union etc did not work is that it is designed to work with a solid shape not a path (unless you vectorize the border of a path)
What you need to do with your shape is really one of 2 things…
- Take each of the paths and join them together so you have a single path for the whole shape then you can just apply a fill to it
2.Recreate the shape another way
I would suggest that option 2 is going to be easier in this case.
To create that shape what you could do is…
- Draw a rectangle.
- Convert the rectangle to a path
- Subselect the bottom left corner and apply a curve to the corner
- Then subselect the top right corner and do the same but change the type of corner to ‘Round 2’ (an inside curve)
- That should give you the basic shape then add a border to match up with your other shapes
Hope that helps