Hi Max,
You are using a Text Box Input widget which is designed to allow a user to modify that attributes value. Since that attribute is an Auto-Number, which is automatically generated by the platform, you get an error.
It sounds like what you actually want to do is set the association to an existing VerkoopOrder / Artikel. This can be done via the ‘Reference Selector’ input widget. This will give you your drop down.
That worked perfectly!
Thanks!