There is a closed case on github with the solution to the problem:
https://github.com/Azure/arm-ttk/issues/424
Links to the templates:
https://github.com/40net-cloud/fortinet-azure-solutions/blob/main/FortiGate/A-Single-VM/createUiDefinition.json
https://github.com/40net-cloud/fortinet-azure-solutions/blob/main/FortiGate/A-Single-VM/azuredeploy.json
Get in touch if you need more help with this issue.
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