Hi,
There is two required fields in partner model/form, ie, Accounts Receivable and Accounts Payable, for which you haven't entered the values, that is why you are getting this warning.
You might not see this fields in form, until you activate the Full Accounting feature, for the users. Go to Settings -> Users and Companies -> Users (make sure that the developer mode is enabled), and tick the Full Accounting Feature. Once the option is enabled for the user, refresh the screen and go to customers form view, you will see this fields now.
To Enable Full Accounting Features:
Fields in the Contact Form:
For Reference:
How To Activate Developer Mode in Odoo
How To Enable Full Accounting Feature In Odoo
Thanks