Accessing Redmine controllers through a plugin
See here :id => @document@document is the variable that contains nil however this should contain some id (like 1,2 or whatever) of required record check it in your controller and once you will fix this issue sure this error will be resolved.
validate route format rake | rails route
send the expected parameters and the correct namespace. If you already have a @document you could use the form_for.
`<% form_for(@document, url: {action:'add_attachment'} ) do %> `