Is it normal for a PhD program to retract or delay financial offer for international student?
Many departments guarantee that their students will be funded got a given period of time, although they can't provide specific details on where the money will come from in advance. That's probably what's going on here; they can't issue the letter yet because they don't know what they should write.

If you can't afford to wait to apply for the visa, then you should contact the international office at the school you'll be attending, as Bill Barth suggests in his comment. They should have plenty of experience issuing such documentation, or helping incoming students to get the information they need for their applications.