I’m going to suggest that you don’t try to use SketchUp, especially SketchUp Free, like AutoCAD. Learn how to use SketchUp the way it was designed to be used starting with the instructional materials at the link John provided.
You can draw a reectangle in the YZ plane starting at the origin and dragging out in the correct plane. Then enter 1,1 if you want it to be 1 unit square. No need to enter coordinates.
Sketchup and autocad are different softwares with a different way to model or draw something, in sketchup you can’t type coordinates to draw a line, it’s way easier, just use the line tool click where you want it to start, then ser the direction you want that line to be drawn, you can lock a direction using the arrows or holding shift, then type the length you want that line to have and it’s done, for rectangles it’s almost te same the only difference is that you must use ; between the length of the rectangle size. Before starting to draw you can lock the plane where you want to draw the rectangle using the arrows, up will lock it on the xy axis, left on the xz axis, right on the yz axis and down will lock it on any referenced plane, if you have a wall at 30 degrees and you want to draw a rectangle at the same angle, put the cursor on the wall then the down arrow and it will be locked to the angle of the wall.
Generally speaking, the first thing I tell someone with AutoCad experience who is trying to learn SketchUp is, “try to forget AutoCad.”
So many acad users have spent years learning to think the way they want you to think. SketchUp is a 3D modeling tool for people who don’t want to learn “cad”. Want to draw a line! Click the Pencil and draw a line. Want to look around your model in 3D space? Use your mouse to Orbit.
The hardest thing for Acad users to accept is that all the learning they did works against them in SketchUp. Try to approach it with fresh eyes. Pretend you are starting from scratch… try to forget what UCS even is!
I disagree with the first part, see ( *). And yes (second part), it’s often way easier to use the drawing tools and take advantage of SketchUp’s inferencing engine.
( *) For instance you can start drawing an edge with the ‘Line’ tool. And with or without clicking the second endpoint of the edge type (say) [6m,3m,7m] and [Enter]. The edge will then end at that location. The current drawing axes and origing will be taken into account. This also works in editing context of a group or component. Then the context’s current drawing axes and origin will be honored. The ‘Text’ tool always referes to the system’s axes and origin when clicking on an endpoint, this contrary to the ‘Tape Measure’ tool. This tool will in turn show the coordinates in the current drawing axes when pointing to an endpoint when examining geometry. So in a way one do have the option to work with UCS (in object’s editing context) and to enter coordinates.