You can fix this by adding an isset check in your TPL file. The error occurs because $cart.subtotals.tax is null but the template tries to access its properties. In your themes/your_theme/templates/checkout/_partials/cart-summary-totals.tpl file, find the line causing the error (around line 77) and wrap the tax-related code with isset(): This prevents the template from trying to access properties of a null value. Clear your cache afterward. The issue typically happens when tax rules aren't properly configured in International > Taxes > Tax Rules.