I’m not sure what you mean by netlify.org I presume that’s just a typo.
If you mean trying to change your site name to bmi-calculate
Then yes, that’s also already taken by this:
https://bmi-calculate.netlify.app/
It’s true that you own that domain name, but it’s also unfortunately irrelevant.
Anyone can set their site name to any name that is available.
(You’re also ignoring the fact that people own bmicalculate.com, bmicalculate.net, etc)
The interface to input your site name is correct when it’s telling you these names are taken, but another way you could check, (and confirm to yourself what is already there), would be to use your browser address bar to hunt for an available name.
By simply checking if sites exist at whateveryouwant.netlify.app
If it shows “Site Not Found” then it’ll be available for you to use.
Here’s an example I just tried with bmicalculate123.netlify.app
