I got you, Steel123.
In QuickBooks Desktop (QBDT), the items that were set up as Inventory cannot be changed to a Non-inventory items. You can only change it to an Inventory Assembly-type item. Please see attached an image below for your reference:

Instead, you’ll want to copy the detail of the item and create a new one. For the old item, you can either make it inactive or delete it. Here's an article for your reference on managing your items: Add, edit, and delete items.
You might also want to check these links for additional reference in tracking your inventories effectively:
Be sure to get back to me if you have other concerns about inventory. I'll be right here to assist you further. Wishing you well.