Amazon reports 'EPR number invalid' for France – what is causing this?

In short

Usually one of these three causes is to blame: the wrong number was entered (France requires the IDU – Identifiant Unique – from the ADEME register, not your contract or customer number from the éco-organisme), the number was entered in the wrong format (the IDU follows the pattern 'FR' + six-digit number + underscore + two-digit scheme code – '01' for household packaging – + four letters, e.g. FR123456_01ABCD), or the registration is not yet active/updated in the ADEME register. Check the IDU in the open ADEME dataset and enter exactly that identifier.