How to Fix TWRP "unable to mount data" Error and Internal Storage showing 0mb?

If you are experiencing TWRP unable to mount storage and internal storage showing as 0 mb, read through this guide for how to fix it.

Reminder

This steps will wipe or format Data partition and data will be reset.

Confirm that internal storage became encrypted when using a legacy screen lock method. Go to Android device’s Settings > Security > Screen Lock and change screen lock method to either Pass or Pin. Create a new one.
Reboot device into TWRP Recovery, enter the pass or pin you just created.
TWRP will attempt to decrypt device’s internal storage. If it succeeds, you should not have further issues.
Reboot device into TWRP Recovery again.
Go to Wipe menu > Advance Wipe.
Select Data and go to Repair or Change File System.
Click Repair File System to see if this fixes the issue. If not, continue.
Go to Change File System.
Select Ext2 and swipe to confirm.
Now switch back to Ext4 and swipe to confirm.
Go back to TWRP main menu, then the Mount menu, and check if your partitions can be mounted now.
If you are still unable to mount your partitions, you need to repair Internal partition, which will most likely wipe your internal storage.