I am having trouble fixing a broken website that is giving me a code to input, but I am receiving an error that I do not know how to fix

Kind of a weird thing to post here, but I am trying to access Yugioh Top Deck and it is sending me to a page that says to put a few line of code into Windows Powershell as an Administrator.

app.rb:

class TopDecks::WebApp  get '/lander' do"Hello World"  endend

I put this in and get back the error message

At line:2 char:15+ class TopDecks::WebApp+               ~Missing 'class' body in 'class' declaration.+ CategoryInfo          : ParserError: (:) [], ParentContainsErrorRecordException+ FullyQualifiedErrorId : MissingTypeBody

I have tried to look at different coding pages about 'class', but I am still unable to figure out what is going wrong. I am not familiar with coding so I am hoping I can get some help

I have not tried typing anything into my Powershell since I use this computer every day and I do not want to risk a wrong code input ruining something unintended