From what I could see from researching this, it is a known problem, which apparently is not fixed.  You can try contacting support.

Resolution for 'account because issue: 'account was disconnected from bank in order to change type but i still get ‘you cannot change the type of this account because it is reserved for Online Banking feature’' available: Yes.

The account because issue or error code is a known issue in Quickbooks Online (QBO) and/or Quickbooks. Support for this issue is available either by self-service or paid support options. Experts are available to resolve your Quickbooks issue to ensure minimal downtime and continue running your business. First try to resolve the account because issue yourself by looking for a resolution described in the article. If it is a complex account because issue or you are unable to solve the issue, you may contact us by clicking here or by using other account because support options.

For more information about resolving issues related to account because in Quickbooks/QBO or to get support for the issue 'account was disconnected from bank in order to change type but i still get ‘you cannot change the type of this account because it is reserved for Online Banking feature’', click the link below: