How do I practise without access to a mainframe?

Vendor learning systems — IBM and partners periodically run free or low-cost programmes with real z/OS access. Availability changes; check what is current.
Emulated environments — open-source emulators run older MVS releases with public-domain system software. JCL, the editor and the batch model transfer well.
Cloud-hosted training LPARs rented by training providers.
GnuCOBOL — a free compiler for learning the language itself anywhere.
Paper practice — reading and explaining JCL, DISP, BIND and pseudo-conversational design needs no system at all, and it is most of what interviews test.