Screen name validation error

Issue

Unable to choose a specific Screename. It could be cause by the following reasons:
1. This specific Screen name is already chosen
2. This Screen Name is not validated because the characters are not according to the screename characters politics.
3. Conflicts with an external authentication service policy like LDAP or Single-signOn

If the Screen name is already chosen, you need to choose another one because this field is unique
You can modify the screename validator politics with the following properties:
- To allow numeric characters on the Screen Name:
  - ```
  users.screen.name.allow.numeric=true
```
- To allow special characters on the Scree Name:
  - ```
  users.screen.name.special.characters=-._
```
If there is an issue with an external authentication service, check their configuration and policy.

You can even auto-generate the Screen Names with the following property:
- ```
users.screen.name.always.autogenerate=true
```
If still need to customice the validation politics Input a class name that implements com.liferay.portal.security.auth.ScreenNameValidator. More information about Screen name validator here.