The opening balance field for sales ledger accounts is greyed out

Hello and Welcome to the Community! 

Once a customer has been created and has transactions posting against them it's not possible to enter an opening balance within their settings.

Instead, to record this, you can create an invoice with the open balance date, with Sales income account selected on the 'Product/service' field and the 'Amounts are' field set to 'No VAT'.

Please get back to us below if you have any Q's!