Unfortunately, we don't support creating a new partnership between Windows Server 2019 and Windows Server 2016. You can safely upgrade a server or cluster running Windows Server 2016 to Windows Server 2019 and any existing partnerships continue to work.
To get the improved replication performance of Windows Server 2019, all members of the partnership must run Windows Server 2019. You must also delete existing partnerships and associated replication groups, then recreate them with seeded data (either when creating the partnership in Windows Admin Center or with the New-SRPartnership cmdlet).