ExistingVestingSchedule

A vesting schedule already exists for the target account and this call cannot add another. Check the account's Vesting storage entry before attempting to set a new vested transfer or schedule.

Declared by the Balances pallet; it classifies to the semantic code already_exists.

Remediation

The object or state already exists; treat the goal as met or pick a different target

The same explanation is available in the terminal: btcli explain ExistingVestingSchedule.