"Ragna's first outing"
This is probably a reference to the Japanese practice of sending a small child on an errand by themselves. It is done at an early age that would be considered dangerously young in the United States. In manga, the parents would typically shadow the child in secret to ensure that they are safe as they negotiate the streets or public transportation. So the joking implication is that Ragna is being treated like a small child on an errand.