Make sure you're in Chrome, version 86 or later. You can check what version of Chrome you're using by typing chrome://version in the URL bar.
Some ad blocking browser extensions may block some of this demo's functionality. Deactivate ad-blocking extensions on the demo pages.
Don't use an Incognito or Guest profile.
Go to flags by typing or pasting chrome://flags in Chrome's URL bar.
Turn on the two flags #conversion-measurement-api and #enable-experimental-web-platform-features.
Disable third-party cookie blocking. In the long term, dedicated browser settings will be available to allow/block the API. Until then, third-party cookie blocking is used as the signal that users don't want to share data about their conversions—and hence that this API should be disabled.