I’m running Incus on Alpine and to get the UI working (after installing via apk add incus-ui-canonical@testing), I found I needed to append this to /etc/conf.d/incusd:
INCUS_UI="/usr/share/incus-ui"
and restart Incus (rc-service incusd restart).
Incus then recognized the UI immediately.
Credit to this post for helping me realize it was an environment variable issue.