encryption – Unable to mount fastboot boot twrp.img backup unable to mount storage?

I was trying to load fastboot boot twrp.img and take backup of stock rom it loaded successfully but while taking backup its not able to take backup its giving error that cant mount storage its not able to select sdcard microsdcard and internal memory to showing 0 free space

I googled and finded it said its due to encryption and recommended to put screenlock pin and use those pinto uncrypt but tiried invain and it said to use wipe and change filesystem to ext2 ext4 but all invain

is there any way to mount and show the internal memory or external sd card and decrypt any suggestions

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My mobile device have

OS Android 6.0.1 Marshellow Processor Qualcomm Family cortex a53
Machine arrch 64 Architecture arm cortex a53 External memory card 64GB
ROM 16 GB MOBILE NOT ROOTED

thank for your suggestion time and advice