Why isn't viewed checkout tracking working?
A common cause of viewed checkout tracking not working is upsell apps and cart drawer apps.
These apps block other apps from being notified when the checkout button is clicked. They do this to prevent other apps from interfering with their bundling and upselling functionality, but it has the unfortunate side effect of preventing even analytics apps from working on checkout button click.
Disabling these apps should restore checkout button click tracking. To confirm this, please try temporarily disabling these apps' app embeds in your Theme Customizer then recording a replay and viewing checkout.
If that resolves the issue, please reach out to your upsell or cart drawer app provider and ask if they have a workaround to prevent their app from interfering with analytics apps.