[Salesforce]INVALID_FIELD_FOR_INSERT_UPDATE, Account: bad field names on insert/update call: AccountId: [ AccountId]

INVALID_FIELD_FOR_INSERT_UPDATE, Account: bad field names on insert/update call:  AccountId: [ AccountId]

上記の解決先を共有します。

public void syncContact (Contact contact) {
  ...
  ...
                    Contact newContact = new Contact();
                    newContact.Id = id;
                    newContact.IsTarget__c = true;
                    update newContact;

  ...
  ...
}