Recover data without correctly installed OS

The custom ROM on my phone has a failure, so it always boots into twrp. Unfortunately it's impossible to reinstall the OS (I always get failure "Error opening (permission denied)). So I want to save a few files I haven't backuped. Is there any way to access the files from twrp?

1. Connect your device to the computer.
2. In "Mount" menu select Data. Then push "Enable MTP".
3. If you are lucky, you could navigate through filesystem of your phone "Data" partition, find your files and copy them to your computer.

Whether this method failed and your phone has microSD connector, you could try copy your files to microSD

1. Insert microSD to your phone
2. In "Mount" menu select Data and Micro SD
3. Start "Filemanager" in "Advanced" menu
4. In Filemanager find your files and copy them to microSD

Also you could read this page https://droidwin.com/transfer-install-files-twrp/