TWRP createTarFork() error 255 when backing up data

I have installed FreedomOS on my OnePlus 5. I'm not sure if this is related to FreedomOS, but when I try to make a backup in TWRP with data, system, and boot, it has an error with createTarFork() shortly after it gets to backing up data. It also says 'read only filesystem' when I try to open Terminal from Advanced and run 'cd /data/media/0/TWRP/BACKUPS/bad7d639 && rm -rf backup-name'. What can I do to fix this and why is this happening?