Enable all the required components

sudo pacman -S thunar thunar-volman gvfs gvfs-mtp gvfs-gphoto2 gvfs-afc polkit

(You may already have some of these — that’s fine.)

Enable the Polkit authentication agent

Thunar uses polkit to ask for permission when mounting drives.
You need a polkit agent running in your session.

If you don’t have a desktop environment, you can install a lightweight agent:

GNOME agent:

sudo pacman -S polkit-gnome

Then add this line to your Niri autostart (or startup script):

/usr/lib/polkit-gnome/polkit-gnome-authentication-agent-1 &