Currently serverless feature not supported by ARM template. Manually changing to serverless from the portal results in an sku name of GP_S_Gen5_1 (choosing the most basic). If I update my fully-working ARM template with this SKU name (instead of S0) for example, I get an error on deployment reporting:

"error": {
"code": "InvalidSkuName",
"message": "Invalid SKU name 'GP_S_Gen5_1'."
}

However this is already the value of the sku name on resource JSON export as a result of the change via Portal.

NOTE: There were other detected changes to the JSON export (from Portal changes), which I also ensured were updated in my ARM template, however the error did not report issues with these values:
- Changed SKU tier to GeneralPurpose (from Standard)
- Changed kind from v12.0,user to v12.0,user,vcore,serverless