You are seeing the result of separation of cash and bank accounts into separate tabs, as described in this release note: [17.8.3] Added "Bank Accounts" tab. As a result of this change, the checkbox is unnecessary. The Convert feature is intended for those who had accounts set up improperly before the switch. See this Guide: https://www.manager.io/guides/11727.
Bank statement import procedures are as described here: Import bank statements | Manager.