It still works for me, in that most textures can be downloaded in the smallest size for free credits. There have been several students recently reporting this problem though. It seems to be down to their anti-abuse system such that it takes 24hrs or so before the free credits are added to new accounts. This is to prevent multiple account creation to get lots of free credits and download loads at once.
So I would give it a day or to and check in again.
There are lots of sites with free textures. The course directions that are causing the problem are usually after a simple photograph of a flat front view of some urban housing. That is not on other sites but really any image you can source, or take yourself, will function