Is 20 feet the maximum altitude for the Levitate spell?

So in my current game, I am playing as an Air Genasi, which gives me access to the Levitate spell (emphasis mine):

One creature or object of your choice that you can see within range rises vertically, up to 20 feet, and remains suspended there for the Duration. The spell can levitate a target that weighs up to 500 pounds. An unwilling creature that succeeds on a Constitution saving throw is unaffected.

The target can move only by pushing or pulling against a fixed object or surface within reach (such as a wall or a ceiling), which allows it to move as if it were climbing. You can change the target’s altitude by up to 20 feet in either direction on Your Turn. If you are the target, you can move up or down as part of your move. Otherwise, you can use your action to move the target, which must remain within the spell’s range.

However when I used it, we came to a bit of a clash in interpreting the rule. The way I read it, it says that I can move them 20 feet up or down on my turn. The way the GM read it however, was that it was to a maximum altitude of 20 feet.

Is there any official errata to clarify this?