Hi there, Pamela.
In QuickBooks Online (QBO), the invoice dropdown is designed to select customers or donors, not vendors, since an invoice is a sales form that tracks money owed to you and is typically associated with a customer.
Vendors, on the other hand, represent expenses and amounts you owe, so you'll have to use the Expenses or Vendors menu to manage them. Therefore, it's not possible to change the field from Customer/Donor to Vendor.
If your goal is to use an invoice to charge an actual vendor (billing the same entity to whom you also pay bills), you can consider entering that vendor as a customer, as a workaround. Although this isn't standard practice since invoicing is primarily associated with income, it is still possible.
QuickBooks allows you to have the same entity as both a Customer/Donor and a Vendor, as long as you use slightly different names (for example, "John" for the vendor profile and "John S." for the customer profile). You can then create an invoice for "John S".
Please let us know in the comments if you have any further questions.
