The “Fixed angle” checkbox is missing in the Rigidbody2d controller in the version (the newest version) of Unity I have. It’s not missing in Mike Geig’s version of Unity in the tutorial I’m watching. It probably it is there in mine, I just don’t know the location.
Could I be told the “Fixed angle” checkbox location in the version of Unity that I have?
It’s now under the Rigidbody Constraints.

There, right?
I’m trying to avoid “the character falling flat on his face”. What should I do if I’m correct about the location? If I had to guess, I would say check the “Freeze Position”-‘Y’.
Freezing position will stop him from moving, so freezing position X will stop movement left-right, and freezing position Y will stop movement up-down. Therefore, I think Freeze Rotation Z is the best option.