Add "vnetRouteAllEnabled": true,

into a block that looks like this inside the Main arm template file:

"properties": {
        "name": "[parameters('webAppName')]",
        "serverFarmId": "[parameters('serverFarmName')]",
        "httpsOnly": true,
        "vnetRouteAllEnabled": true,
        "minTlsVersion": "1.2",
        "siteConfig":

to find Main arm template file - check deployorder

make sure the variables in parameter file are called in main file

Find JSON view in Portal.azure.com under Web App - overview, top right corner - JSON View

for networking blocks like this :

"vnetName": {
      "type": "string"
    },
    "vnetRG": {
      "type": "string"
    },
    "subnetName": {
      "type": "string"

Do this instead in the main file:

"vnetIntegrationSettings": {
      "type": "object",
      "defaultValue": {
        "vnetRG": "",
        "subnetName": "",
        "vnetName": ""
      }
    }

Do this in Param file:

"vnetIntegrationSettings": {
      "value": {
        "vnetRG": "DV-DV-AsmtRegistration-RG",
        "subnetName": "CC-SUB-DV-AsmtRegistrations-API-10.165.194.16_28",
        "vnetName": "CC-VNET-DV-PAAS-10.165.192.0_19"
    }
  }

Setup

Troubleshooting