What’s happening in a majority of these cases is that TWRP is unable to decrypt your internal storage – basically, TWRP does not properly handle encrypted storage. Another cause could be corruption of the /data partition, and TWRP being unable to find it. If you are experiencing TWRP unable to mount storage, and internal storage showing as 0mb, read through this guide for how to fix it.
How to Fix TWRP Unable to Mount Storage
Warning: This option will remove all the data contains in your mobile. Please make a back-up of your data before proceeding to the next step we will not be responsible for any loss of your data.
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