There, it’s described as a legacy package, so not actively being used.
Try installing it first,
pamac build libnm-glib-nosystemd
and then see if you still get the error.
Edit:
OK, didn’t know that it’s not a dependency. Saw that it is a ‘NetworkManager client library’ so it made perfect sense to me, thanks for pointing it out @Yochanan! My mistake for not investigating further.
I’ve had zero issues with mullvad on manjaro and I know all the versions of it are maintained by a member of the manjaro team so can’t understand why it fails to install