Sometimes, you may see User is 0 in GA4 reports, but other metrics have data, then there may be the following reasons:
Users is Active Users
You can see that Engaged session is 0, it is not an active user, so Users is naturally 0.
If there is too little data, the data threshold is triggered. In order to protect user privacy, the data will not be displayed, so Users is 0.
When a threshold is applied to a report or exploration, the data indicator changes color. You can hover over the indicator to see more information. The indicator can also help you identify when sampling is applied.
Data not processed
If you are looking at the data of the current day, Users may also be displayed as 0 when the data has not been processed: