That looks correct. I was able to copy and paste the JSON text from your attachment and get back “1 min” using these blocks:
Is your API “Get” call returning an error (green error block)?
Are you doing anything else with the DurationStep1 variable after this that might affect its value? What happens if you replace the “set app variable DurationStep1” block with a “from Label1 set Text to” block?
Are you initializing the DurationStep1 variable as blank (no connected block) or as a particular type (text string, list, etc.)?