Can’t customise look of invoice in your app.

Good to see you here in the Community, @vihan-soni.

Whether you're accessing your QBO account via desktop or mobile app, you can customise your invoices if you have the Plus or Essentials version. However, if you're using Simple Start, this is the possible reason you're unable to use the Custom Form Styles module. For more information, check out this article: How to Customise Invoices. 

You'll want to consider our higher versions so you can personalise your forms and add the info that matters to you. To view the comparison and pricing chart between the three Online versions, visit this website: QuickBooks Online Plans. Once you've chosen the right plan for you, you can upgrade your subscription.

You can visit this website: Customise Your Sales Forms. In this link, you'll find steps on how to create, design, and edit your template, and personalise your email message.

I'll be right here to help if you need anything else. Have a good day, @vihan-soni.