Hi @sramdeen
As @QFBeth mentioned, you can add a bit of code to your account if you have a Power User Subscription.
If you go to Account Settings >> Design Customisation >> Advanced HTML/CSS Customisation, and add the following to Global CSS, this will show the nominal code box on all invoices (new and old):
.page-sales-create .categoryDetailsContainer { display: block !important; }
The result would look like this for all invoice lines:
