DIAGNOSIS This is almost always an app firmware mismatch or a glitch in the scheduled brew data being sent from your phone to Keurig's servers.
FIX Force-close the Keurig app on your phone and reopen it. Check for app updates in your device's app store and install any available. Then, delete the scheduled brew and recreate it from scratch. Ensure the brewer's clock is set correctly in the machine's own menu (Settings > Time).
IF THAT DIDN'T WORK Log out of the app completely, then log back in. If the error persists, delete the brewer from the app and re-pair it via the QR code on the bottom of the machine.
HEADS UP Scheduled brew requires the brewer to be in standby (plugged in, powered on, but not actively brewing). It will not work if the brewer is in the middle of a descale lockout or has an 'Add Water' alert active on its display.