It's possible that the tracking number has not yet been activated in the system. Sometimes it takes a little time for the tracking information to become available. In such cases, you might need to wait a few hours or even a day before the tracking number starts working.
Double-check to ensure that you have entered the tracking number correctly. Even a minor error, such as a missing or wrong digit, can prevent the tracking number from being recognized.
Sometimes, there might be technical issues or maintenance activities on La Poste's tracking website temporarily preventing tracking numbers from working.