Perhaps this is trivial, but I want to be sure: It is essential that your cursor be within the cell when you hit SHIFT+ENTER. In your screenshot, you have the line below the cell, and if you hit SHIFT+ENTER in this scenario, nothing will happen.

If you arrow key up into the cell, does SHIFT+ENTER still fail?

If the cursor is blinking horizontally it is not in the cell above. It is waiting for new input into the next cell. Click somewhere around 2+3 and hit SHIFT+ENTER

What happens with the menu command Evaluation > Evaluate Notebook? This should evaluate everything in the notebook file.