PAUSE SCRATCH TAPE NEEDED ON X'280'

I've been following your tutorials on installing 360/DOS on hercules and I'm having the following problem. When attempting to assemble the supervisor (SYSGEN01) I get the following message:

// JOB SYSGEN01   ASSEMBLE NEW SUPERVISOR
00.00.49
// PAUSE SCRATCH TAPE NEEDED ON X'280'

... any ideas?