Thank you for showing that screenshot. The "Item Number" lookup column in your SharePoint list "Purchase part change list" does not come from an Excel workbook. The "Item Number" lookup column in your SharePoint list "Purchase part change list" is using the column "PRTNUM_01" as a key column from the SharePoint list titled "Purchase Part Item List" as a source.
You need to find the SharePoint list "Purchase Part Item List" and add any new items using the key column "PRTNUM_01" and they will be visible in your "Item Number" lookup column on your current SharePoint list "Purchase part change list".

The best way to search for the SharePoint list "Purchase Part Item List" will be through searching the 'Site Contents' of the current SharePoint site.
You find the Site Contents by clicking the gear ⚙️ icon like before and then selecting "Site Contents" which will take you to another page.
Site Contents
When you get to the Site Contents page, find the other list titled "Purchase part item list" and this is where you will add new items so that they can be looked up in your "Purchase part change list" in the Item Number column.
Site Contents
Let me know if this solves your issue. I'm not sure how the Excel workbook is involved, but I know for sure that your "Item Number" lookup column in the "Purchase part change list" is directly linked to the "PRTNUM_01" column in the list "Purchase part item list" as the source.