Hi M Bader,

Yes, you can add a customer which is also a vendor in QuickBooks Online however, you'll need to add  a character or symbol on one of the names so you can save it without any error. 

To add a customer: 

1. Go to Sales tab and select Customers. 
2. Click New customer.
3. Enter or add customer details.
4. Select Save.

To add a vendor: 

1. Go to Expenses tab and select Suppliers. 
2. Click New supplier.
3. Enter or add supplier details. Make sure to add a character or symbol if you're adding the name as a customer.
4. Select Save.

Additionally, you may find these articles helpful: 

Error: The name supplied already exists. Another customer, supplier, or employee is already using th...

Feel free to message us back if you need further assistance. Have a great day!