If you're trying to allocate an account that's not listed yet in the account lists, you'll need to add a new account manually. Here's how:
Go to Accounting on the left panel.
Within the Chart of Accounts tab, click the New button.
Select an account type, detail type, and enter the name of the newly added account.
Then, Save and Close.
Once done, you'll be able to record transactions associated with this account by following these steps:
In the Chart of Accounts tab, find the account where you want to enter the transactions. Then, click View Register.
Click the Add drop-down arrow and select the type of transaction.
Enter the amount and other required details.
Make sure to select the correct dates.
Then, Save.
In addition, some accounts were created automatically in the program and serve an important role in your accounting. These are special accounts in QuickBooks Online. You can check this article for additional information: Manage default and special accounts in your chart of accounts.
To keep your transactions organize, you can use these articles as your guide: