Credit card “not supported” when I try to sign up

I am trying to sign up with a Discover IT Student card (it is the only credit card I have). However, it tells me that my card is not supported. I looked on their support pages and double check everything I entered, and I still cannot figure out why it won’t work and if it is a problem that can be resolved. Additionally, I contacted support, but their responses are not helpful at all. Has anyone else had this problem? (Also, do they accept debit cards? Different sources seem to be giving contradicting information on this.)