@Antik

Under the Configuration of POS you need to define the Pos Payment Methods i.e 'Journals'. Here while defining the journal under the 'Cash Register' tab you need to set the flag 'Cash Control' (checkbox to True).

While creating a new Point of Sale you need to add this journal into it. Now when you will try to start your pos session you can provide the denomination for Opening Capital.

Hope this helps !!.