By default, a fresh SambaPOS install will not print a receipt on payment. A receipt is only printed when you click Print Bill.
You need to add the action to print receipt after payment received, you can follow the instructions here under the “How to automatically print bill after payment received?” section:
Normally you would use the Print Bill button when you want to print a receipt to a customer before they have paid, e.g. when they ask for the bill / check.