Previously when working with this platform I faced a similar issue. The root issue was caused by the Pi Server using Kerberos authentication by default. Currently you will be unable to get Kerberos to work on the Flexy and additionally it is unsupported in PostMan.
It is pretty easy however to enable either BASIC or Anonymous authentication. I personally would recommend Basic as it is pretty easy to utilized and still provides some level of security.
Inside of the Pi Server, launch the PI System Explorer as an administrator.
Locate your API element and select the System Configuration.
Inside of the system configuration area you should see a field labeled: AuthenticationMethods, this value is a string array that can contain your configured/allowed authentication methods. Dobule click this value.
Now you will probably see Kerberos there by default. Click Insert in the popup and add Basic or Anonymous then click add. I personally recommend deleting Kerberos at this point.
Update the values and then send the Pi Server for a quick restart.
After doing the above you should be able to authentication using Basic authentication in Postman or by using this syntax in a browser: username:password@piserverurl