How do I summon an armor stand with a steve head, leather armor, and posable body parts?
You put the Pose tag inside the fourth entry in the ArmorItems tag. To solve this, just move }] after {SkullOwner:Steve}.
In general, I recommend putting commands into Notepad++ or another editor that shows bracket pairs, that helped me finding the issue.
Or if that doesn't help, try removing parts of the command until it works, then try finding the shortest command that still has the issue. Most of the time you'll already find the issue that way, otherwise people at least have to check less to give you an answer.