E:/ failed to mount in TWRP (bootloop)

I would need some more help with my G4. I flashed a sound mod and my LG G4 got stuck in a bootloop, so I decided to restore backup, which wasn’t possible because of the e/ failed to mount.

I factory restored the phone, but because of the problem I still cannot install the custom ROM again.

Any help would be appreciated. I can enter TWRP and download mode, so I guess flashing a stock KDZ is still possible.

Thank you