You have at least two problems. First, you have made this as a 3D sketch and it needs to be a 2D sketch. You will have to recreate it as a 2D sketch. To help with that, try selecting all the geometry in the 3D sketch and using copy (Ctrl+C) and then create a new 2D sketch, select the front plane, and paste the geometry there (Ctrl+V). Then you can position it and dimension it and delete the original 3D sketch. Then a second problem I see is that this is not a single closed contour. You have at least one extra solid line going down the center of the part. You need to either delete that or convert it to construction geometry.