I just made the call using postman and the json returned doesnt match your json structure. It returns a list with a single object.
Here is the postman call where I got the json to make json structure
Then I made this json structure and import mapping
Then created a microflow that looks like this
Here is my test project where I showed the result in a pop up page.
Hope this helps!
Edit: Alternatively you could use the community commons random strong password java action.
That is awesome. Thank you so much. I did think it was the JSON structure but hadn't tried adding the . My understanding of JSON is still a little flaky. I will check out the community's generator as well.