"I can confirm that."
Translation:Das kann ich bestätigen.
"Dass" only means "that" when it comes before a sentence (a subordinative clause, if I'm not mistaken). For example:
"I know that you speak german"
"I know" : main clause "that" subordinative conjunction "you speak german" subordinate clause
In the case of our sentence, we must use "das", in order to replace another acusative object.
Please correct any mistakes I made. Hope that helps :)