There is an error in the yaml… see the update, you need to add the mac address for it to work.
network:
version: 2
ethernets:
enp0s3:
match:
macaddress: ???
mtu: 1500
set-name: enp0s3
bridges:
br0:
interfaces: [enp0s3]
dhcp4: true
parameters:
forward-delay: 15
stp: false