This can work fine. You are not allowed to change the auto number, but you are only referencing it and reading the value.
All you are probably missing is reading rights for this attribute. Check the access-right of Bestelling/Bestelnummer for having read-rights for the Project-role that you are signed in with.
Hi Aydin,
Autonumbers are automatically generated by mendix which we cannot edit in pages or microflows.
Use integer instead of autonumber for ordernumber to get the orderline.
Thanks,
Bharathidasan