from the errors it looks like the interface 'Inside' is not mapped to anything on the FortiGate you're trying to install to.
I would assume the SNAT fails as well because an interface is not in place/configured properly, but without knowing the config details of the SNAT entry it's hard to say if that fails due to the 'Inside' interface, or for an unrelated reason.
Can you check the interface object 'Inside' in the Policy&Objects tab, and verify what interface it is mapped to on your FortiGate? That might provide some insight.
+++ Divide by Cucumber Error. Please Reinstall Universe and Reboot +++
Were you able to sort out your issue? I'm running into a similar issue where we don't have a "MGMT" interface on the Fortigate, in the Provisioning Template, or the Policy Package but it's throwing an error saying MGMT is an invalid mapping.
Edit: Sorted it out! I had a Normalized Interface rule for MGMT which had a Per-Device mapping configured for the device.
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