If that happens to anyone, just turn the phone off with long power button press and try again. Jumpdrive is using an older kernel which had a bug that made it sometimes not recognize internal storage, but it should work after at most a few tries.
Before launching that script, check lsusb for an entry like:
Bus 001 Device 106: ID 1fc9:012b NXP Semiconductors i.MX 8M Dual/8M QuadLite/8M Quad Serial Downloader
If it’s not there, it means that either your phone isn’t actually in flash mode, or there may be something wrong with USB cable/connection.