See solution below.
Explanation
1. Sign in to the portal,
2. Choose template Deploy-lod9940427
3. Select Edit template, and then paste your JSON template code into the code window.
4. Change the ASddressPrefixes to 10.0.0.0/24 in order to support only 256 total IP addresses.
addressSpace":{"addressPrefixes": ["10.0.0.0/24"]},
5. Change the firstSubnet addressprefix to 10.0.0.0/26 to support only 64 total IP addresses.
"subnets":[
{
"name":"firstSubnet",
"properties":{
"addressPrefix":"10.0.0.0/24"
}
6. Select Save.

7. Select Edit parameters, provide values for the parameters that are shown, and then select OK.
8 Select Subscription. Choose the subscription you want to use, and then select OK.
9. Select Resource group. Choose an existing resource group or create a new one, and then select OK.

10. Select Create. A new tile on the dashboard tracks the progress of your template deployment.
References:
https://docs.microsoft.com/en-us/azure-stack/user/azure-stack-deploy-template-portal?view=azs-1908
https://docs.microsoft.com/en-us/azure/architecture/building-blocks/extending-templates/update-resource