Create new accounting journals to be used for opening balances:

GL Opening Balances

Journal type = 'Miscellaneous'.
Code=OPACC
Description = 'Opening Balance Journals'

Debtor Opening Outstanding Balances

Journal type ='Sales'
Code=OPINV
Description= 'Opening Customer Invoices/Credits'

Creditor Opening Outstanding Balances

Journal type = 'Purchase'.
Code=OPBIL
Description = 'Opening Creditor Bills/Credits'

TIP: Using short codes all starting in "OP" allows them to be easily identified and filtered.

Below: Example of new journals created. 
 

For each bank account that will need to be imported, ensure there is a corresponding bank journal created. Check the configuration on each bank journal to ensure the Outstanding Receipts account has bee created and applied to all lines under the Incoming Payments tab.