Sinatra doesn’t know this ditty.

Try this:
# in main.rb
class GrapesRV
  get '/%D0%B8%D0%B7%D0%BC%D0%B5%D0%BD%D0%B5%D0%BD%D0%B8%D1%8F_%D1%81%D0%B5%D0%B7%D0%BE%D0%BD%D0%B0_2021-2022' do
    "Hello World"
  end
end