You've accessed an existing process flow but can't make any changes on the canvas - there are no options to add shapes.
What you can and can't do with a process flow depends on the version you're viewing. If there are no options to add shapes to the canvas, it's likely that you're viewing a deployed version or an inactive version of the process flow. Only the draft version of a process flow can be updated.
Try one of the following:
Switch to the draft version of the process flow
Use the copy to draft option to create a new draft version of the process flow
For more information please see our Process flow versioning page.