I want enable acount number but im getting an application error 90 , please help

I'll share with you the two options to fix the application error 90 when turning on the Enable account numbers feature in the settings, @abeda-abrahams. 

If you're using a web browser, its stored cache may be full. It may cause unusual behavior in QuickBooks Online (QBO). That's why you received an error when checking the Enable account numbers box. 

First, I'd recommend accessing your QBO account via a private browser. It's a great way to isolate browser-related issues since this doesn't store data in the cache. Here are the keyboard shortcuts depending on the browser you're using: 

• Mozilla Firefox or Microsoft Edge: CTRL + Shift + P
• Google Chrome: CTRL + Shift + N
• Safari: Command + Shift + N

Once signed in, go back to the Chart of Accounts section from the Advanced tab. Then, check the Enable account numbers box, click Save, and choose Done (see the screenshot below). If you're able to do so, switch back to your regular browser. Then, clear its cache to remove its junk files. That way, it'll function efficiently. If not, you can use another supported browser. 

After that, you can assign any number to your accounts. It helps you organize your chart of accounts in QBO. For detailed steps, see Step 2 through this article: Use Account Numbers. You'll also learn more about the number ranges and account types that follow generally accepted accounting principles (GAAP).

If you're using the QBO desktop app, I'd suggest clearing its cache. It restores the application to its default settings to help fix the error 90. I'll guide you how. 

1. Go to Help from the top menu. 
2. Choose Reset App Data.
3. Allow a moment for the process to complete, and then close the app.
4. Be sure to right-click the app and Run as Administrator when re-opening it.

As always, you can visit this website: QuickBooks App for Windows and Mac. It provides you more information about opening a new window, transaction, and report from the top menu. It also contains links to speed through common workflows using keyboard shortcuts, multiple windows, and taskbar preview. 

Please let me know if there's anything else you need. I'll gladly help, @abeda-abrahams.