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.
This steps will wipe or format Data partition and data will be reset.
Step 1: 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.
Step 2: Reboot device into TWRP Recovery, enter the pass or pin you just created.
Step 3: TWRP will attempt to decrypt device’s internal storage. If it succeeds, you should not have further issues.
Step 4: Reboot device into TWRP Recovery again.
Step 5: Go to Wipe menu > Advance Wipe.
Step 6: Select Data and go to Repair or Change File System.
Step 7: Click Repair File System to see if this fixes the issue. If not, continue.
Step 8: Go to Change File System.
Step 9: Select Ext2 and swipe to confirm.
Step 10: Now switch back to Ext4 and swipe to confirm.
Step 11: Go back to TWRP main menu, then the Mount menu, and check if your partitions can be mounted now.
Step 12: If you are still unable to mount your partitions, you need to repair Internal partition, which will most likely wipe your internal storage.