Hi Samuel H, 

The Reason Why this validation Pop-up arises, When we try to close the session from the Backend,

not from the POS Screen, as you can in the below image.

Because there is no Payment Line Selected for Specific Journal, in POS Payment Methods.

insert image_1

1) As you can see in below image that I will Make a Payment From 'Bank' Payment Methods From 

   POS Product Screen.

    Insert image_2

2) Go to POS > Configuration > Payment Methods. After that click on your specific method. as I have 

    selected Bank in below image.

    Insert image_3

3) After that, click on Journal, and you can see that I have set the Payment Line for Journal 'Bank'.

   Once it is set, then user can close the session.

   Insert image_4

   Insert image_5

In case of more help, you can reach to us.

Thanks & Regards,

Email: odoo@aktivsoftware.com           

Skype: kalpeshmaheshwari