If you run odoo 10, I know how to fix it. This problem happen when you login with a user not administrator. When you create new user, at the same time, odoo 10 will create a contact that has an attribute "is a customer" = True. You need to change this attribute to False by uncheck this checkbox. 

Hi 

This is due to the user who create customer is no rights for customer creation 

To Provide Rights 

Activate developer mode 

Goto Settings> User > contact creation (tick this check box)

Hope you will find the answer

Foch is rigth: 

If you run odoo 10, I know how to fix it. This problem happen when you login with a user not administrator. When you create new user, at the same time, odoo 10 will create a contact that has an attribute "is a customer" = True. You need to change this attribute to False by uncheck this checkbox. 

To uncheck "is a customer", you may install contacts module -> go to contacts->select your user > go to sales and purchases > and under Sales uncheck "is a customer"

maybe connection to your server is loss

make sure the wifi icon in your pos is green

Hi Mathieu, I had the same problem in mine. It was because I was recording the customer with the TAX ID as is in my country, (11212343456). Odoo requieres that the TAX ID have a format with the coutry code in the front (EC11212343456).