sometimes, you can only use ' be able to'.
such as:
1. Kids might not be able to discriminate between good and bad.
[so when there is a 'might, maybe' in a sentence, it's better to use 'be able to' rathert han 'can']
2. I'm sorry to inform you that our department will be unable to provide service this afternoon due to a power outage.
[here is a 'will', you can't say: will can't... ]
For certain reasons, I'll be unable to attend the meeting.
You will be able to come, won't you?
3.You need to be able to work as part of a team.
[here is a 'need to']
and so on...