Once you click "Point of Sale", you will see the following image:
You can create invoice by following these steps:
You can add Product to the cart from the inventory or by searching items from the search bar.
New product can also be added by clicking "new".
You can also create invoice from the tickets tab by searching for tickets and creating an invoice against it.
You can add Customer from the top right corner.
Once customer and items are added, click "checkout" to collect payment and complete an invoice.