"Items attached to a large number of transactions can't be processed."

Welcome to the Community, mimran-nvt-gmail.

Thanks for posting your question here. I want to make sure you're able to attach items in QuickBooks Online.

Looking into this I was able to see this is a current issue going on within the QBO system. The product engineers are aware and working to get it resolved as quickly as possible. I recommend reaching out to a member of the QuickBooks Online Support Team to be added to the list of affected users. Once you're added you'll receive email notifications regarding updates and the resolution. Here's how to get in touch with the team:

1. Click the Help (?) button.
2. Select either tab to get started:
  - Assistant: Get quick, personalized answers. Select a suggested option, or type a question or topic you need help with. If you decide you need further help, you can still Talk to a human.
  - Search: Search the QuickBooks Online knowledge base directly, or select Contact Us and choose a way to connect with us:
    a. Start a chat with a support expert.
    b. Get a callback from the next available expert.
 

The following article provides these steps if you ever need them again in the future: QuickBooks Online Support

Please feel free to reach back out if you have any other questions. I'll be here to lend a hand.