The error message "WorkspaceInstaller is not supposed to be executed in databricks runtime" indicates that the WorkspaceInstaller class or function is not designed to be run directly within a Databricks runtime environment. This is a general limitation of the Databricks Labs installation process, not specific to UCX.

From the context provided, it appears that the WorkspaceInstaller is intended to be used in a different environment, possibly a local development setup or a different execution context outside of the Databricks runtime.

Can you try to install CLI in your local machine and test it from there?

I am running this through Databricks web terminal. How would it work from local?

Databricks CLI is designed to work in your local terminal and connect to your workspace. 

The error is mentioning that the command is not allowed to be run in a Databricks cluster, as you are using the web terminal of a cluster in your workspace it will fail.

You can follow steps in https://docs.databricks.com/en/dev-tools/cli/index.html to have the CLI installed on your local machine