Select the dropdown corresponding to your operating system, below.
source <install_path>/Vitis/2020.1/settings64.sh vitis
Note: Regardless of OS, if Vivado is open, Vitis can also be launched through the Tools → Launch Vitis toolbar option.
Upon launching Vitis, a dialog will appear where a workspace must be chosen. The workspace is the directory where all of the projects and files for the application being developed will live. If a folder that does not currently exist is chosen, it will be created. Choose a workspace and click Launch to finish launching Vitis.