Did notpunch down or use first rise. Let dough rest 10 min. then shaped and let rise in pan per yeast directions.
You still didn’t provide any visual observations and these are critical to diagnosing your problem. Directions regarding making bread can rarely be followed precisely and produce consistent results. For instance, kneading for 5 or 6 minutes may or may not produce adequate gluten development. It depends on your mixer, your hydration, the protein level of your flour, and the existence of any gluten inhibitors in your formula. The only way to know is to perform a gluten window test, also known as the window pane. If you don’t know what that is, you can Google it. If your gluten is inadequately developed, your bread will not rise as high and will be over proofed more easily.
Secondly, 1 hour may or may not be enough time to proof your dough. Depends on the quantity of yeast, hydration, temperature, availability of yeast food, and the health of the yeast. If you did not develop the gluten adequately, the dough might be over proofed by the time it doubles and it may result in a brick when baked. You can use the finger poke test to determine when the bread is ready for baking.
I’m going to make the assumption that the dough actually did double before you baked it, which would indicate that the yeast was alive, had adequate food, and the room was warm enough to keep the yeast active. In that case, I’d place my bet on over proofing the dough.
For what it’s worth, the recipe you’re using isn’t a very good one. With no primary fermentation, the bread won’t produce much in the way of flavor. And with only one rise, it probably won’t be as light and fluffy as it could be with a punch down or two. I recommend that 1) you get a good bread book, like Rose’s Bread Bible, 2) learn about the window pane test, and 3) learn about the poke test.