Found this question but it doesn't actually answer the question of why this doesn't work:
ln -s /storage/folder_a/dir_of_stuff_I_want /storage/folder_b
operation not permitted.
ln -s /mnt/storage/folder_a/dir_of_stuff_I_want /mnt/storage/folder_b
Same error. This is internal data (not trying to link to external SD like most of the other questions), I'm assuming ext4, not sure why this fails or do I need to specify the path some other way?
Phone is a Galaxy s8, running 7.0. Terminal Emulator is Terminal Emulator for Android.