Welcome to the Community, sachin-awasthigr.
In QuickBooks Online (QBO), activities like the option to customize while printing invoice is available depending on what type of user access you have in the account.
It's possible that the access is limited to Customers reason why it doesn't allow you to customize invoices. To fix this, have the admin of the account verify and correct your type of access. You can share this article with them: Add and manage users in QuickBooks Online.
If you require any further information about types of users and permissions in QuickBooks Online, check out this article: User roles and access rights in QuickBooks Online.
Once everything is set up, you can refer to this article for steps on how to customize your invoice and other sales forms in QBO: Customise invoices, estimates, and cash memos in QuickBooks Online
I'll be around in case you need further assistance. Place a comment below, and I'll get back to you.
