To create host-named site collections or to create site collections within a content database the self service creation mechanism cannot be used. Due to this restriction appropriate permissions are required. By default the AppPool user of the WebApplication is used. If your AppPool user does not have the required permissions, you can specify a user with the appropriate permissions. Note that you may have to configure new 'Credentials' within the 'CredentialStoreConfiguration'.

public RunAsUserConfiguration RunAsUserConfiguration