The problem is not a "decrypt" you need to disable the "encryption" as well.I remove pin and i tried to backup.
Same problem.
How can i decrypt my data ?
TWRP can decrypt the file in encrypted (almost latest TWRP now support decrypt A13)
So if do a backup the /data partition that encrypted, change ROM, restore the data, then the new ROM will be failed to decrypt the encrypted data from previous ROM.
That's why TWRP not allowed you to backup encrypted data.
There's a two solution for this case.
1. Backup all your internal memory file to PC/Flash drive/Micro SD.
So. You can transfer back the data after change your ROM.
2. Disable Force Encryption (DFE). You will lose all data from internal memory (/data partition)
You need to format /data, install DFE, and stay unencrypted.
You need to flash DFE every updating/change ROM to keep disable forced encryption