You may have set up sales GST/VAT rates in QuickBooks that's why it's asking you for Tax Code upon entering receipts. You'll have to select Out of scope of Tax from the Amounts are drop-down so you can save the sales receipt successfully. Let me walk you through the process.
Before we start, please make sure that the item you're adding isn't taxable to avoid issues when filing your tax to your tax agency.
Here's how to exclude your receipts for a Tax Code:
Click the Plus (+) icon.
Select Sales Receipt.
Choose your customer from the Customer drop-down.
Enter the necessary information.
In the Amounts are drop-down, select Out of scope of Tax.
Click Save and close.
I've attached a screenshot below for your visual reference.
Here are some articles you can read on for more details: