Just to clarify, the workflow you show in your screenshot is a backend workflow?
If that’s the case, you’re trying to change a variable in the frontend directly from a backend workflow, but this isn’t possible. The backend has no context of the frontend.
If you want to save a response from an API called through the backend into a frontend variable, then you would need to create a frontend workflow that has:
one action that triggers the backend workflow
then a “Change variable value” action that updates the variable with the value returned by the backend workflow.