In order to show Item details tab on Expense form, first, you need a Plus plan. Essentials and Simple Start does not have Item details tab feature.
If you have a Plus plan, you will need to check the following two setups:
1) Gear settings for Expenses – Gear at the top right corner > Your Company > Account and Settings > Expenses > Bills and expenses > Show Items table on expense and purchase forms > On > Save.
2) Product/Services information setup form – Gear at the top right corner > Lists > Products and Services > Select Add new button (or for an existing item, click “Edit” under Action column) > Purchase information > Select “I purchase this product/service from a vendor” checkbox > Save and close.