Because it doesn't make sense. Sentence A can't be true is sentence B is true.
A: Pick an amount that makes you feel anxious.
B: If you aren't anxious...
If you aren't anxious, then you didn't "pick an amount that makes you feel anxious". I think I see the point, but the logic is incorrect and needs to be clarified.
The point, as I see it, is that you should find an amount of work that you can comfortably complete rather than trying to do too much. If I were to write it, I would say:
Pick an amount of work that you think you can complete before the end of the day. If the amount makes you feel anxious, you are trying to do too much.