What to do if you don't see data in your automatically created GA4 property
If a new GA4 property has automatically been created for you, but you are not seeing any data in the new property, it's because your Universal Analytics property tag could not be reused. This happens when your tag is not the Google Tag (gtag.js) or analytics.js.
In this case, you'll need to manually add the Google tag or configure Google Tag Manager.
Additional information
You will need to retag your site if you use ga.js, urchin.js, or advanced analytics.js features including: using Tasks, changing any default cookie configuration or behavior, changing client id, overriding any of the document (page) parameters or the referral source, disabling advertising features, or changes to property processing rate.