From translatewiki.net

Invalid user name specified, or missing parameter in {{User|User name|
home = Project:Interwiki (optional) |
homeuser = Home user name (optional) }}

The documentation of this template is included from its /doc subpage. [Edit]
This template is used to link to the user page and talk page of users. It takes two parameters, but only one is necessary.
  • {{User|~~~}}</nowiki>
  • {{User|~~~|Display name|
    home = Project:Interwiki |
    homeuser = Home user name }}
  • |1=Local user name (mandatory) – The exact username of the user in question on translatewiki.net, with correct capitalisation.
  • |2=Display name (optional) – An alternate display name, if that is desired.
  • |home=Project:Interwiki (optional) – A short interwiki prefix (before : and the generic User: namespace prefix) to the home wiki where the user has his or her main talk page, such as commons (for Wikimedia Commons), wikipedia:m (for Wikimedia MetaWiki), wikipedia:mw (for MediaWiki), wikipedia:d (for Wikidata), wikipedia:en (for Wikipedia in English), wikipedia:v:de (for Wikivoyage in German). This should start by one of the interwiki codes which are listed on Special:Interwiki with the status "Forward: yes" (further codes indicated after a colon, such as an interwiki language code for a specific linguistic edition or subproject, are not checked locally on translatewiki.net: they are passed as is and resolved on the forwarded site of the first interwiki code given).
  • |homeuser=Home user name (optional) – The alternate user name used on the home wiki, needed only if distinct from parameter |1= (parameter ignored if |home= is not specified)