Hi,

In the one2many line the option for adding a new line, ie, Add an item, is missing as there is no create permission for the user in the corresponding model.

Suppose if you take the case of the sale.order and sale.order.line, if for the logged in user group have no create permission for the model sale.order.line , then from the user interface the add an item will get disappeared as there is no access for the creation.

To solve this, what you can do is that, as stated in the one of the comment above, give necessary access/permission for the logged in user group. You have to grant the create permission for the user group. Also you can click the Become Super User button, once you activate it, you can see the add an item button without giving the access rights. This is not permanent solution. To activate the super user mode, activate developer mode and click the Debugger button near logged in users name in the menu bar and click the Become Super User.

Thanks