请告诉她我不能跟她去看电影了:Please tell her I can't go to the movies with her because I'm in a hurry this time, but welcome to the next date.
请告诉她我不能跟她去看电影:Please tell her it's impossible for me to go to the movies with her and stop trying to date me.
10:00 we have agreed to meet later, 15:00 I realise we can't meet, 17:00 "Please tell her we can't meet": 请代我向她致歉,我事去不了了,下次再约吧。To go back on an already decided date, we usually apologize and welcome another date, lest the other person think we have a problem with her.
10:00 AM, we discuss meeting alter but don't make any firm decision, 15:00 I realise we can't meet, 17:00 "Please tell her we can't met": 请替我告诉她,我有事去不了了,约会取消吧。There is no need to apologize because the date has not yet been decided, just articulate your decision.