First of all forgive me if you open the file and find errors because I am still a SketchUp novice. 
I am building this frame and I’ve arrived at a point where I cannot understand how to fill the selected area.
This has to be done actually for all four sides.
Can someone please help me?
Chuwi Hi12 (cornice).skp (1.6 MB)
This is a good application for Follow Me. Draw a rectangle as a path and a cross section of the frame molding. Here I’ve created a path and profile. Then I selected the rectangle as the path, got the Follow Me tool (I used a keyboard shortcut) and clicked on the profile.
As it is, your arcs at the corner are not on the right plane for the miters so they’d need fixing first.
What are the blocks for?
Are the dimensions correct?
Why is your model so far up above the ground plane?
First of all thanks for helping. 
That model didn’t have to be in so I just forgot it to delete.
The blocks are made to be able to fix and remove that frame from a wall tablet chassis (the model I forgot to delete hehe) and I made it because I wasn’t good enough to create clips.
I will try now using the Follow Me method. 
