You create an App Service plan named Plan1 and an Azure web app named webapp1. You discover that the option to create a staging slot is unavailable. You need to...

The app must be running in the Standard, Premium, or Isolated tier in order for you to enable multiple deployment slots.
If the app isn't already in the Standard, Premium, or Isolated tier, you receive a message that indicates the supported tiers for enabling staged publishing. At this point, you have the option to select Upgrade and go to the Scale tab of your app before continuing.
Scale up: Get more CPU, memory, disk space, and extra features like dedicated virtual machines (VMs), custom domains and certificates, staging slots, autoscaling, and more.
Incorrect:
Scale out: Increase the number of VM instances that run your app. You can scale out to as many as 30 instances
Reference:
https://docs.microsoft.com/en-us/azure/app-service/deploy-staging-slots
https://docs.microsoft.com/en-us/azure/app-service/manage-scale-up