Follow the following steps to record your opening stock balance.
Make sure you have your inventory account ready under Chart of Accounts.
Key in your accounts' opening balance as usual under Chart of Accounts > ... (Beside + New button) -> Opening Balance.
Enter your closing stock value into your Inventory account, say 31/12/2020.
Proceed to create a stock adjustment entry following the closing date. (Products & Services > Stock Adjustments)
Enter your stock balances one by one or import an Excel file with the button on the top right.
Click Save.
Due to system limitations, having completing the steps above will adjust the value of your Inventory account as well as the Inventory Adjustment account with the stock adjustment amount, do the following steps to overcome this limitation:
Create a Journal Entry following your opening stock value based on your closing date. (Accounting > Journal Entries)
And you're all done!