Processes in Microsoft 365 for setting up Office apps, redeeming product keys, and activating licenses.
When you purchase a Microsoft 365 subscription from Curry's they should always provide your with the product key card, if not, if you provided the cashier with your Email address, check your Email for the product key.
If you still cannot find the product key, then the cashier is incorrect in telling you to contact Microsoft with your receipt to get the product key, you need to go back to the store, ask to speak the manager to get the product you paid for.