TWRP password decrypt storage
That TWRP requires a password is because your /data is encrypted, which is default since Android 6.

To permanently decrypt the filesystem, you need to format /data and flash a "disable force encryption" package. Be aware that formatting /data will delete all your files, including photos, music, etc. in your internal storage.

Also be careful when flashing other ROMs or applying OTAs, as most of them will make your /data encrypted again and you'll have to format it again. You need to flash the "force encryption disabler" every time you flash another ROM/OTA.