就是can't。英语中很多不要直译,意译也很重要。
I can't do that.或I don't know how to do that.
I have no ability of doing sth.
I am unable to do sth.
I am not capable of doing sth.
I am afraid I can not do it/I am afraid I`m unable to do it.
I do not know how to do that.