On page https://developer.paypal.com/docs/multiparty/checkout/fastlane/integrate/ the "Integrate front-end" section shows some wrong HTML code
<script>
src="https://www.paypal.com/sdk/js?client-id=<CLIENT_ID>>&components=buttons,fastlane"
data-sdk-client-token="<SDK_CLIENT_TOKEN>"
</script>
The "src" and "data-sdk-client-token" need to be inside the first tag
<script
src="https://www.paypal.com/sdk/js?client-id=<CLIENT_ID>>&components=buttons,fastlane"
data-sdk-client-token="<SDK_CLIENT_TOKEN>">
</script>