Here is what I think you are missing:

When you book online, you must tick the "use rewards points" box. It is located to the right of the box that displays your Marriott Rewards number. When you do this, your search will only display rooms that are available for points. Some of those rooms may also require an additional cash payment, so read carefully. Also please note that those cash payments are in the currency of the country you are visiting, not US dollars.

In the future, you should be aware that you can book for points before you have earned enough points to cover your stay. Just make sure you earn enough points before the stay begins. Marriott will debit your account about one week prior to your stay. This allows you to book on spec almost an entire year in advance.