They disable all the purchasing options on the test site, because who needs 35,000 robux on the test site when it will just be deleted on a random basis.
@CLIPSE
Actually, on test, we're allowed to buy robux on "John Doe's" account. We can get free robux, up to 300 on one account per 24 hours. I got Dominus Empyreus 1/26 on test, I should know.