You can't have two conjugated verbs together without a conjunction between them:
Pueden trabajan would translate to: Can you, you work? It doesn't make sense, so the unconjugated verb infinitive is used instead in its pure meaning:
pueden trabajar - Can you work...
Yes. Easy for me to think.of it as "are you able to work" That way the infinitive to work sticks out in my mind.
Normally duolingo accepts "you all" for Ustedes, why did it not in this case
Perhaps DL is encouraging the use of the less regional phrase "all of you."