Legally you should 'hold' a PPL but there are plenty places who will enroll you on atpl, assuming you ae simply waiting or licence issue. When I did my groundschool, 2 out of the 8 students had just finished their PPL and were waiting patiently for the brown book to arrive. The School didnt have a problem with it. They were more than happy to sign people up and take their money!
But as whirlygig says, phone and ask the school!!!