Hello and welcome to PSG.
Here's another approach.
Since I don't know your level of experience with Ps, I will just summarize.
Open a new document.
Use the Rectangular Marquee Tool to make a selection of the upper (wall) section of the rectangle.
Fill with color.
On a new layer, do the same for the lower (level surface) section.
On new layers, create gradients on the upper (wall) and lower (level surface).
Create a new layer.
Set up your Brush Tool for a stroke path operation.
Select your Pen Tool.
Create a path on the line where the upper and lower sections meet.
Right click and choose stroke path.
Go back to your Brush Tool and adjust for a larger softer stroke.
Create a new layer and repeat the above stroke path operation.
Select the uppermost layer and create a Stamped Visible layer. Shift + Alt/Opt + Cmd/Cntrl + E
Convert the stamped vis layer into a smart object
Add a Gaussian Blur smart filter..........adjust to taste.
This gives you a good starting point if you want to add other effects.
Good luck!