Not your mistake but on pos.order action default show tree view not form view

and  you have done only on form view not on tree then you can show create button on tree.

i have added code try it.

FORM

<record model="ir.ui.view" id="misc_view_pos_pos_form">

<field name="name">misc.pos.order.form</field>

<field name="model">pos.order</field>

<field name="inherit_id" ref="point_of_sale.view_pos_pos_form"/>

<field name="priority">25</field>

<field name="arch" type="xml">

<xpath expr="//form" position="attributes">

<attribute name="create">false</attribute>

</xpath>

</field>

</record>

TREE

<record model="ir.ui.view" id="misc_view_pos_pos_tree">

<field name="name">misc.pos.order.tree</field>

<field name="model">pos.order</field>

<field name="inherit_id" ref="point_of_sale.view_pos_order_tree"/>

<field name="priority">25</field>

<field name="arch" type="xml">

<xpath expr="//tree" position="attributes">

<attribute name="create">false</attribute>

</xpath>

</field>

</record>