If it's the "famous bug" on macOS, depending GIMP version and your mac version, you won't see the selection, but... it's selected, (better to update your GIMP as the bug was fixed)
To add to what rich2005 wrote, as visual reference, to help you see what you've selected, tick the check box "Draw mask" in the tool options, a magenta color will fill selection (while clicking the mouse, and disappear on mouse release)
Now to be sure if it's that bug we are speaking or not (no visible marching ants), please try to select something on an image, then copy (CMD+C) and paste (CMD+V) you should see a new layer called floating selection in the layer's dialog/stack above the original layer.
Then Click on the + (layer) icon at the bottom of the layer's stack to merge the selection to a new layer > is it what you did selected?
To add to what rich2005 wrote, as visual reference, to help you see what you've selected, tick the check box "Draw mask" in the tool options, a magenta color will fill selection (while clicking the mouse, and disappear on mouse release)
Now to be sure if it's that bug we are speaking or not (no visible marching ants), please try to select something on an image, then copy (CMD+C) and paste (CMD+V) you should see a new layer called floating selection in the layer's dialog/stack above the original layer.
Then Click on the + (layer) icon at the bottom of the layer's stack to merge the selection to a new layer > is it what you did selected?
Patrice