Red Rock of Ravannis

What does 'Artifacts can't be carried here' mean?

Artifacts which share a location with the Red Rock are no longer carried. They are not dropped, they just stop being carried, so the effect will still apply to artifacts which cannot be dropped. These artifacts also cannot be targeted by the Pick Up ability.

How is this resolved if either the artifact or the bearer cannot be pulled?

If the bearer of the artifact cannot be pulled, but the artifact can, the Red Rock pulls the artifact out of the bearer's hands. If the artifact cannot be pulled, neither the bearer nor the artifact are pulled.