Invoicing in the app is a little bit different from the web version, userkunal-lexicon. 

I'll guide you through the steps on how to apply itemwise discount using  the QBO mobile app. Here's how:

1. Tap the Plus icon and select Create invoice.
2. Look for Add product or service and tap on it.
3. Look for the item or service that you'll be adding in the invoice, then tap on it, and then select Add.
4. Select the discount item that you set up and tap on it.
5. Tap Add.
6. Enter all others details in the invoice and tap Save. 

Here's an article about this for more details: Discount By Line Item On Invoices And Sales Receipts. 

Feel free to check out these articles as well about using the app for more details and reference:

• QuickBooks Online Mobile App FAQ
• Customise Invoices On The QuickBooks Online Mobile App
• Track The Status Of Your Invoices In The QuickBooks Online App 

Keep me posted if you need more help or when looking for something in the program.