If you click on chart of account from accounting dashboard on top it will redirect accounts list where you can set opening credit and debit for each accounts. If you allready close it or open it, then it will not show you. In that case you can show that tree view with little cutomization and then fill your values.

Its possible to access initial Balances from the dashboard until it is disabled by the user: Initial setup

You can get the configuration steps back by editing view 'account.setup.company.form' and remove the 'invisible =true' part of the field. 

\https://drive.google.com/open?id=1mvN2x0HlaT8KGBJ6rMygECaPtcCvRBdW

Then you can uncheck  the 'hide setup-bar' in the account configuration. the configuration steps are shown again on the dashboard and you can open and edit the chart of accounts with the initial balances, or open the initial balance move lines. 

Have you checked if you have an account more than once in your inital balance, maybe once with a debit value and once with a credit value? Or perhaps you have a debtor with a credit value? 

This particular error is caused by having a single line that has both a debit and a credit. This can happen at any time, not just during opening balances. 

If you are trying to import a line with say a credit of 10 and a debit of 6, you can either balance them off and keep the credit of 4, or have two lines, one with a credit of 10 and one with a debit of 6.