Hi,

From Odoo13 the Boolean fields is customer and is Supplier, have been removed. In v13, it is handled in different ways, system will automatically decide the customer and vendor based on the history(no of customer and vendor invoices).

You can create a record in the contacts menu and if you create a sale/invoice for this contact, you can see it will come under the customer filter.

You can check the explanation given here: How To Handle Customers and Suppliers in Odoo13

Also read this commit message: https://github.com/odoo/odoo/commit/8626257bc7b9ee772006eb17d12f56344e594f25

Also two new fields customer_rank and supplier_rank is introduced, which are Computed fields to order the partners as suppliers/customers according to the amount of their generated incoming/outgoing account moves.

Thanks

You can create a customer from the Customer menu.  If you create a contact from the Contacts app they will not be a customer, and there's no standard option to set them to be a customer!

It's quite simple to create a Server Action to do this. 

 The other change in Odoo 13, is that a contact does not have to be a customer to be selected on a sales order.

Similar question: https://www.odoo.com/forum/help-1/question/where-are-is-customer-is-vendor-checkboxes-in-odoo-13-contacts-157566