Accessing Google Calendar using Oauth via Ruby Sinatra app - ruby

Question

I'm building a Slack bot, with a backend Sinatra app. The function of the bot is to add and read entries from Google Calendar.
I'm using session and environment variables.
I'm unable to open up the authorization webpage for Oauth and get access to user's Calendar data. Please help!
Here's my code:
#The redirect_url entered in Google Console.
#Google Oauth redirects to this endpoint once user has authorised request.
get '/oauthcallback' do
client = Signet::OAuth2::Client.new({
client_id: ENV['CALENDAR_CLIENT_ID'],
client_secret: ENV['CALENDAR_CLIENT_SECRET'],
authorization_uri: 'https://accounts.google.com/o/oauth2/auth',
redirect_uri: "https://agile-stream-68169.herokuapp.com/oauthcallback",
code: params[:code]
})
response = client.fetch_access_token!
session[:access_token] = response['access_token']
redirect url_for(:action => :calendars)
end
#The Authorization url which checks if credentials are valid.
get '/authorize' do
# NOTE: Assumes the user is already authenticated to the app
user_id = request.session['user_id']
credentials = authorizer.get_credentials(user_id, request)
if credentials.nil?
redirect authorizer.get_authorization_url(login_hint: user_id, request: request)
end
end
#METHOD: Initialising the API by creating a Calendar Service
def intialize_api
$service = Google::Apis::CalendarV3::CalendarService.new
$service.client_options.application_name = ENV['CALENDAR_APPLICATION_NAME']
$service.authorization = auth_calendar
end
def auth_calendar
client = Signet::OAuth2::Client.new({
client_id: ENV['CALENDAR_CLIENT_ID'],
client_secret: ENV['CALENDAR_CLIENT_SECRET'],
authorization_uri: 'https://accounts.google.com/o/oauth2/auth',
scope: Google::Apis::CalendarV3::AUTH_CALENDAR_READONLY,
redirect_uri: "https://agile-stream-68169.herokuapp.com/oauthcallback"
})
redirect client.authorization_uri.to_s
end