You're trying to create a pack on Win7. As Eileen and Geoff have mentioned, you create the pack in YEM and then move it to the PSR-SX900 either by USB key or by wireless, and then import it using the pack functions on the PSR.
You need YEM to do this, but the YEM-v2.70 won't work on Win7. So you'll either need to create the pack on Win10, or get YEM 2.53 ~ 2.60 from somewhere. If you're in a real bind for this, contact me off-line and I'll send you a copy of 2.60. But I'd recommend you move to Win10 and do it there.