TemplateError (index 58729 out of string)

Could someone please explain to me the meaning of this error?

ActionView::TemplateError (index 58729 out of string) on line #2 :

1: <%= form_remote_tag(:update => "update_form",
2: :url => { :action => :add_product}) %>

Could someone assist me with this problem?

Just a thought, but if the method in your controller is called
add_product, do this instead: :url => { :action => 'add_product' } )

I'm not sure what the error is or if chainging the : to single quotes
will fix it. It's my best guess.

Try add product as string not as symbol