most auto trans with a manual shift option will electronically prevent an upshift or downshift to a certain gear until certain criteria are met, like traveling at a high enough speed that that gear would be appropriate without engine being in proper power range to handle. the same is true with downshifts, you can't manually shift the trans into first gear when traveling at 55mph. if i remember correctly both 5th and 6th gears are less than a 1:1 ratio, overdrive gears would only be appropriate at highway speeds. to prevent engine lugging they would likely lock you out of those gear if you are accelerating and also likely not until you are going fast enough. i would go test out what gear it seems to let you shift into those gear for myself but i already put my car into storage in prep for the coming snow.