How to debug a ConflictError when adding an access policy to a key vault using an ARM deployment

I had the same error, I guess that it is because you are trying to modify the same item "key vault" in parallel. This change fixed my problem:

"mode": "serial"

"copy": {
    "name": "<name-of-loop>",
    "count": <number-of-iterations>,
    "mode": "serial" <or> "parallel"
}

During deployment, you can request that additional information is logged during a deployment. In powershell, set the DeploymentDebugLogLevel parameter to All.

New-AzResourceGroupDeployment `
  -Name exampledeployment `
  -ResourceGroupName examplegroup `
  -TemplateFile c:\Azure\Templates\storage.json `
  -DeploymentDebugLogLevel All

Then you can examine the request content or the response content.

(Get-AzResourceGroupDeploymentOperation `
-DeploymentName exampledeployment `
-ResourceGroupName examplegroup).Properties.response `
| ConvertTo-Json

For more details regarding troubleshoot deployment errors, you can refer to https://docs.microsoft.com/en-us/azure/azure-resource-manager/resource-manager-common-deployment-errors