Spanish phrase
Avísame si no puedes ~.
Meaning
Let me know if you can't ___.
When to use it
Asks to be told if the other person cannot do something. Informal request often used for schedule changes or absences.
Example dialogue
Avísame si no puedes venir mañana.
Let me know if you can't come tomorrow.
Vale, te aviso si surge algo.
Okay, I'll let you know if anything comes up.