ERROR: This page is using an unprefixed version of template {{Documentation}}

Create {{Wp/ng/Documentation}} instead and replace all occurences of {{Documentation}} in Wp/ng/ prefixed pages by that.
You may like to check 3 templates in Category:Wp/ng/Templates using unprefixed template Documentation first to remove all occurences from those templates which will fix the pages containing those templates too.
[create] Template-info.svg Template documentation