The full message ends: "Pick a later day, or change the post times."
A piece with scheduled posts cannot move to a day that would put those posts in the past.
What to do
Move the piece to today or a later day. On the calendar, drag it to the day, or open the ⋯ menu next to the piece and tap Move to date.
Or change the post times: open the piece, tap Publish, and under Posts tap Cancel on each scheduled post. Then move the piece and schedule its posts again with Schedule, a Date and a Time.
When you tap Unschedule on a piece with posts queued, Studio asks you to confirm, and the queued posts are cancelled.