this is the full output
dnf system-upgrade download --refresh --releasever=28
Before you continue ensure that your system is fully upgraded by running "dnf --refresh upgrade". Do you want to continue [y/N]: y
Ultima verifica della scadenza dei metadati: 0:00:00 fa il mar 01 mag 2018 20:24:38 CEST.
Errore:
Problema: nss-pem-1.0.3-6.fc27.i686 has inferior architecture
- nss-pem-1.0.3-6.fc27.x86_64 does not belong to a distupgrade repository
- problem with installed package nss-pem-1.0.3-6.fc27.i686