Well I sure love to hate to see you guys, lol.. Trying to FILLET lines but the program is telling me unable to do so due to non coplaner lines. FLATTEN command is unknown in AutoCAD 2013. My UCS is set on WORLD... Okay, gurus.. What am I missing?

Amar,
FLATTEN is an Express command, Express/Modify/Flatten Object. if you dont have the express menue its unknown command. the way around this is to check the elevations for both line (Z) make them 0 and then use fillet.

You can set the elevations of your lines opening the properties pannel but if you select for example an other thing like an arc you won't be abble to select the Z extremities. I propose you to use the following sequence :
Move all the objects from 0,0 to 0,0,1E99 and then move all the objects from 0,0,1E99 to 0,0,0.
Do it with the general UCS and beware of typing "E" and not "e".
It works with all the object types.
To verify the good effect take a 3D viewpoint like front or left and you will see that all your lines are at Z:0.
Please tell us if it's OK or else...

I think we're good now. I selected the entire floor plan, clicked PROPERTIES, toggled through the drop-down menu (i.e., lines, polylines, etc.) and made all the Z elevations to "0". FILLET worked like a charm. Thanks, all!!