Jellyfin doesn't always fill out that list. When you click "OK" what happens? If Jellyfin can't actually access that folder there will be a pop message stating that.
Jellyfin 10.10.7 (Docker)
Ubuntu 24.04.2 LTS w/HWE
Intel i3 12100
Intel Arc A380
OS drive - SK Hynix P41 1TB
Storage
4x WD Red Pro 6TB CMR in RAIDZ1