Sitecore SXA and JSS site creation is managed through Powershell scripts. Here is the Powershell script location that populates this dialogue
/sitecore/system/Modules/PowerShell/Script Library/JSS SXA/Scaffolding/Functions/Get-NewJSSSiteDialog
If you check the code to validate the site, you will find something like this.
if ($forbiddenSiteNames -contains $siteName -eq $true)
{
$variables.siteName.Error = $([Sitecore.Globalization.Translate]::Text([Sitecore.XA.Foundation.Scaffolding.Texts]::SiteWithThatNameAlreadyExists))
continue
}
-ValidatorParameters @{forbiddenSiteNames = (Get-ForbiddenSiteName $SiteLocation)}
Now, if you will see how it gets the forbiddenSiteNames
, you will get the code like this.
$siteItemTemplateId = [Sitecore.XA.Foundation.Multisite.Templates+_BaseSiteRoot]::ID.ToString()
$forbiddenSiteNames = [Sitecore.Sites.SiteManager]::GetSites() | % {$_.Name}
$SiteLocation.Children | ? { [Sitecore.Data.Managers.TemplateManager]::GetTemplate($_).InheritsFrom($siteItemTemplateId) } | % { $forbiddenSiteNames += $_.Name }
$forbiddenSiteNames | Select-Object -Unique
Try debugging the script in your Powershell window and you will see the list of all sites and then check on which one it is failing and the validation is getting failed.
Hope this will give you some context to identify your issue.