Yep, with that twrp it's impossible for me to format data too. If this happen. then you have two ways to do it:
First: you can try to format data from fastboot with command "fastboot format userdata", but i personally never used that command, so you will use it at your own risk.
Second: You can grab other twrp boot image, but there are two versions of twrp, for pie, and for Q, you must try which one is working now for you. (because it depends from which version of android you are running, it's kinda complicated, but i discovered that LLama's LOS 17 and stock pie android is compatible with twrp for pie, and JOSHIX's LOS 17 and this rom is compatible with twrp for Q)
For Q
twrp_universalQ
OR
For Pie
twrp_universalP
When you will get these twrp, then (read carefully) you must BOOT (not flash) this twrp.
With boot command, nothing will be replaced, it's one time only operation, and after reboot everything's booted as it was earlier.
Go into fastboot and type in cmd:
fastboot boot twrp_universal.img
OR
fastboot boot twrp_universalQ.img
output should be like that:
Code:
D:\LG>fastboot boot twrp_universalQ.img
Sending 'boot.img' (65536 KB) OKAY [ 1.491s]
Booting OKAY [ 5.107s]
Finished. Total time: 6.677s
it depends which one is compatible.
If you boot img, and screen will stop for longer moment at LG G7 ThinQ logo, then press and hold start button and Vol- to reboot device.
Go into fastboot and try boot second file.
From one of these twrp, you will be able to format userdata with no problem.
After format, you can hit reboot, power off. Now you can go to twrp which you flashed with that ROM, and install ROM without formatting data.
For me it worked. I hope it will work for you too.