You control at least one additional creature that does not have Pattern of Rebirth attached to it.
Yenna does not have summoning sickness.
({2} magic symbol) available.
Steps
Activate Yenna by paying ({2} magic symbol) and tapping it, creating a token copy of Pattern of Rebirth attached to any creature you control except the creature that has the nontoken Pattern of Rebirth attached to it, then untapping Yenna and causing you to scry 2.
Activate Ashnod's Altar by sacrificing the creature that has the token copy of Pattern of Rebirth attached, adding ({C} magic symbol) ({C} magic symbol) .
When the creature dies, Pattern of Rebirth triggers, causing you to search your library for a creature card and put it onto the battlefield.