Answer from Salesforce support was to include the ForceComPlatform value in the Features array.
Something like
{
"orgName": "Demo Company",
"edition": "Enterprise",
"features": ["ForceComPlatform"],
"settings": {
"orgPreferenceSettings": {
"networksEnabled": true,
"offlineDraftsEnabled": true,
"pathAssistantsEnabled": true,
"s1DesktopEnabled": true
}
}
}
This created a scratch org for me with 1 Salesforce Platform license.
Hope this helps!
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Thanks!