i can't share documents with external users

hello and sorry for my bad English :-)

I have a lot of SharePoint website. It is modern SharePoint site (communication type) in O365

I have librairies on this websites and when i want to share documents with external share, i have this message "not allow by organisation rules". In internal, it is OK

i dont understand why !  in O365 administration, it is allow to share with external users with Anonymous link for 365 days. If i look also in collection site parameters it is also allow, same configuration.

Why i can't do it ?

thanks