In the bottom right corner of AutoCAD, click on the upside down triangle. Check that there is a tick next to Paper/Model. If there is not, then select it.
Now at the bottom of your screen, where ORTHO, OSNAP, etc is there will be a button which will say PAPER or MODEL depending on if you are in paperspace or modelspace/viewport.
If you are in a viewport and it says model, then just click...