No wandering thoughts does not mean not thinking.

We think of the task that needs to be done;

but when completed we do not dwell on it,

do not linger on failures or successes.

Hearing that we should not have wandering thoughts might sound like we should not be thinking at all. But actually it means we should not have improper thoughts. Having something to do, clearly we need to think about how best to do it. These are proper thoughts.

If people criticize what we did, we need to consider what was said and how best to proceed. Was the criticism valid? Did it not really apply? More proper thoughts.

But what if we then keep returning to thoughts of the criticism? These are wandering, improper thoughts. Why? Recollecting our past successes depletes our good fortune. Recollecting failures plants the wrong kind of seed: embarrassment, guilt, anger, and so on. Both distract us from what we are now doing as well as waste our energy.

Do not linger on the past. Learn from it, move on, and pay attention to the present.