NonDefaultComposite

The account cannot be killed because its composite account data is not in the default state. Check the account's System.Account entry; all balance and data fields must be default before the account can be removed this way.

Declared by the System pallet; it classifies to the semantic code invalid_argument.

Remediation

Check the argument values against the operation schema

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