Many software like Docker or fail2ban won’t work out-of-the-box if you’re using now default nftables in Debian Buster.
To revert back to iptables, run the following :
update-alternatives --set iptables /usr/sbin/iptables-legacy update-alternatives --set ip6tables /usr/sbin/ip6tables-legacy update-alternatives --set arptables /usr/sbin/arptables-legacy update-alternatives --set ebtables /usr/sbin/ebtables-legacy
To cancel the change and use nftables :
update-alternatives --set iptables /usr/sbin/iptables-nft update-alternatives --set ip6tables /usr/sbin/ip6tables-nft update-alternatives --set arptables /usr/sbin/arptables-nft update-alternatives --set ebtables /usr/sbin/ebtables-nft