Azure Cloud Shell Instance Failed to provision

That error usually means Cloud Shell is trying to initialize against a subscription reference that is no longer valid for your signed-in context.

I would try these steps:

1. In the Azure portal, switch directory and subscription explicitly from the top-right directory/subscription filter, then reopen Cloud Shell.
2. Open Cloud Shell settings and reset/recreate the Cloud Shell storage if it is pointing to an old subscription or deleted storage account.
3. Check whether your user still has at least Reader access to the subscription and enough rights to the storage account/resource group used by Cloud Shell.
4. If you have multiple tenants, sign out/in or use an InPrivate window to make sure the portal is not keeping a stale tenant/subscription context.
5. Try Cloud Shell from https://shell.azure.com and select Bash/PowerShell fresh.

If the storage account used by Cloud Shell was deleted or the subscription was moved/disabled, the quickest fix is often to remove the old Cloud Shell storage configuration and let Cloud Shell create a new storage account in a valid subscription.

Docs:
https://learn.microsoft.com/azure/cloud-shell/overview
https://learn.microsoft.com/azure/cloud-shell/persisting-shell-storage