Treadly 2 and Treadly 2 Pro have an auto-alignment feature.
Your unit will arrive with the belt completely aligned. Over time, the belt will shift slightly to the right or the left, as it will align according to your steps (many people are leaning gently either to the left or the right with their steps). That is ok and would not cause any damage to the unit.
There are situations that the belt is aligning too much to one side (see photos), and that can cause unwanted fractions between the belt and the frame itself (you might hear noises coming from the belt, or it could cause an error message, as the unit is not functioning as it should).
To Align the belt, please review the following demonstration video:
Please note: Start with only 1/4 circle spin as instructed, to avoid shifting the belt too much to the other side (If that will happen, you will need to align the belt from the other side. Because the alignment is always done by screwing the blot clockwise on both sides, overdoing it will over tighten the belt, and then you'll need to start over by loosening the belt.