Save the file to you computer, upload it here. My guess is sloppy modeling.
Most likely the curve is not in the same plane as the other edges. Share the .skp file so we can help you diagnose the exact issue and fix it.
It’s not possible to be sure from a screenshot, but if, as it appears, the bottom is along the red axis and the two ends are parallel to the blue axis, then the top curve must also be entirely in the red-blue plane else it is not possible for it to form a face with the bottom and ends. All edges of a face must lie in the same plane.
This is the File
Untitled~~ copy.skp (471.0 KB)
So it is as I guessed. The curve on the top is not in the same plane as the edges at the ends or on the bottom. You can see that easily if you group the geometry and examine the bounding box.
Generally modeling the shape so you get the face is easier if you start with a larger rectangle and then ensure you get the On Face indicator when you draw the arc.
If you are trying to model a curved surface and intend the arc to be drawn out of the plane of the other edges you will have to stitch between the arc and the bottom edge. There are extensions that can do that very quickly such as Curviloft which is available from Sketchucation.
I’ve attached a version of the model with a curved surface made from your existing geometry.
Untitled~~ copy.skp (407.2 KB)
Side note: You have a lit of reversed faces in the model. You should correct those before you go further with this model. There should be no exposed blue back faces.
By the way, the file you shared has two tildes in the filename. This indicates you’ve been working on the backup instead of the original file and then you’ve shared the backup of the backup. Make sure when you open an existing model file that you are opening the original and not a backup.