Startup Debian 9 error: `Failed to start Raise network interfaces`

Remove the /etc/network/interfaces.d/setup file then edit your /etc/network/interfaces as follows :

auto lo
iface lo inet loopback

Save and reboot

The man interfaces:

INCLUDING OTHER FILES

  Lines  beginning  with  "source" are used to include stanzas from other

   files, so configuration can be split into many files. The word "source"
   is  followed  by the path of file to be sourced. Shell wildcards can be
   used.  (See wordexp(3) for details.)

In your case you are using the /etc/network/interfaces.d/setup to configure the network instead of /etc/network/interfaces

Lines beginning with "allow-" are used to identify interfaces that should be brought up automatically by various subsytems. This may be done using a command such as "ifup --allow=hotplug eth0 eth1", which will only bring up eth0 or eth1 if it is listed in an "allow-hotplug" line. Note that "allow-auto" and "auto" are synonyms. (Interfaces marked "allow-hotplug" are brought up when udev detects them. This can either be during boot if the interface is already present, or at a later time, for example when plugging in a USB network card. Please note that this does not have anything to do with detecting a network cable being plugged in.)

Just putting a # in front of source /etc/network/interfaces.d/* in the /etc/network/interfaces file, so it doesn't reference the /interfaces.d/setup file solved it for me. Everything working fine. The solution is kinda logical.

Thank you all for the help!