You need to enable the account numbers from the Accounts and Settings to add an account with the account number from the Chart of Accounts. I'm here to guide you on how.
Sign in to your QuickBooks Online (QBO) account.
Go to the Gear icon, then click on Account and Settings under Your Company.
Click on Advanced at the left pane, then go to the Chart of Accounts section.
Enable the account numbers from there, then mark the box beside Show account numbers.
Hit Save, then Done.
When adding an account, you'll be able to fill out the account details, name, including the account number.