Is there any way to import a list of transactions under a specific account for e.g. I want to import under the Retainage Account.

The option to import a list of transactions under a specific account isn't currently available in QuickBooks Online (QBO), Sammy. However, you can manually add them directly to your register. I'll guide you on how to do this.

Before doing so, please note that not every account lets you add transactions directly. If you're able to do so, you can follow these steps:

1. Go to Transactions and select Chart of accounts.
2. Find the account you want to review. If you're new to QuickBooks, select See your Chart of Accounts first.
3. Select Account history from the Actions column.
4. At the very top of the list, select the Add journal entry, Add deposit, or Add cheque dropdown. Tip: The menu can be hard to see - it's just above the most recent transaction. You can also press Ctrl + Alt + N. 
5. Select the type of transaction you want to add.
6. Fill out the fields to complete the transaction.
7. When you're done, select Save.

Moreover, your Account history lists all your past and current transactions associated with the account. If you need to review your transactions or make quick edits, you can refer to this article to guide you: Transactions in the account history in QuickBooks Online.

Additionally, we'd recommend reconciling your accounts regularly (every month) to track your income and expenses. If you need assistance with your reconciliation, you can check out this article: Learn the reconcile workflow in QuickBooks.

If there's anything else that you need when adding transactions to your account history, let us know so we can help you further. We in the Community are always here to provide you with a solution.