Thanks for asking question! Could you please confirm on App Service plan you are using? As the app must be running in the Standard, Premium, or Isolated tier in order for you to enable multiple deployment slots and each App Service plan tier supports a different number of deployment slots. To find out the number of slots your app's tier supports, see App Service limits.
If the swap operation takes a long time to complete, you can get information on the swap operation in the activity log. On your App's resource page in the portal, in the left pane, select Activity log.
You can create slot by using below sample command from Azure CLI.
az webapp deployment slot create --name MyWebapp --resource-group MyResourceGroup --slot staging
after creating slot you can swap using below sample command:
az webapp deployment slot swap -g MyResourceGroup -n MyUniqueApp --slot staging --target-slot production
Further you can use App Service Diagnostics feature which helps you to troubleshoot your app with no configuration required. When you run into issues with your app, App Service Diagnostics points out what’s wrong and guide you to the right information to more easily resolving the issues.
Go to Azure Portal > Select your App Service > on left side of menu click on Diagnose and solve problems > select configurations and Management > select check swap operations from left menu.
You may also refer to below links might be helpful:
Please let us know if issue remains.