You would need to make sure that Jellyfin can write to that folder on the hard drive. Either as the owner or in the group that owns it. Writing to /dev/shm is the no additional config way to write to ramdisk.
Jellyfin 10.9.11 (Docker)
Ubuntu 24.04 LTS
Intel i3 12100
Intel Arc A380
OS drive - SK Hynix P41 1TB
Storage
3x WD Red Pro 6TB CMR in RAIDZ1 (JF Library)