ruby version error when pushing to heroku - ruby-on-rails

Question

I am running into an error when trying to push my app to Heroku. I have pasted my gemfile below for your reference.
The Error:
An error occurred while installing ruby-2.3.1 remote: ! remote: ! This version of Ruby is not available on Heroku-18.
Gemfile:
source 'https://rubygems.org'
git_source(:github) { |repo| "https://github.com/#{repo}.git" }
ruby "~> 2.3.1"
# Bundle edge Rails instead: gem 'rails', github: 'rails/rails'
gem 'rails', '~> 5.2.2'
# Use sqlite3 as the database for Active Record
# Use Puma as the app server
gem 'puma', '~> 3.11'
# Use SCSS for stylesheets
gem 'sass-rails', '~> 5.0'
# Use Uglifier as compressor for JavaScript assets
gem 'uglifier', '>= 1.3.0'
# See https://github.com/rails/execjs#readme for more supported runtimes
# gem 'mini_racer', platforms: :ruby
# Use CoffeeScript for .coffee assets and views
gem 'coffee-rails', '~> 4.2'
# Turbolinks makes navigating your web application faster. Read more: https://github.com/turbolinks/turbolinks
gem 'turbolinks', '~> 5'
# Build JSON APIs with ease. Read more: https://github.com/rails/jbuilder
gem 'jbuilder', '~> 2.5'
gem 'bootsnap', '>= 1.1.0', require: false
gem 'bootstrap', '>=4.0'
group :development, :test do
# Call 'byebug' anywhere in the code to stop execution and get a debugger console
gem 'byebug', platforms: [:mri, :mingw, :x64_mingw]
gem 'sqlite3'
end
group :development do
# Access an interactive console on exception pages or by calling 'console' anywhere in the code.
gem 'web-console', '>= 3.3.0'
gem 'listen', '>= 3.0.5', '< 3.2'
# Spring speeds up development by keeping your application running in the background. Read more: https://github.com/rails/spring
gem 'spring'
gem 'spring-watcher-listen', '~> 2.0.0'
end
group :test do
# Adds support for Capybara system testing and selenium driver
gem 'capybara', '>= 2.15'
gem 'selenium-webdriver'
# Easy installation and use of chromedriver to run system tests with Chrome
gem 'chromedriver-helper'
end
group :production do
gem 'pg'
end
# Windows does not include zoneinfo files, so bundle the tzinfo-data gem
gem 'tzinfo-data', platforms: [:mingw, :mswin, :x64_mingw, :jruby]

Answer 1

heroku stack:set heroku-16, running this command in your terminal will help you.

Answer 2

Change the Heroku Stack for Ruby -v 2.6.2
I used the following command to fix it.
heroku stack:set heroku-16
Reference: upokary.com

Answer 3

Here you can find a list of Ruby versions currently supported on Heroku:
https://devcenter.heroku.com/articles/ruby-support#ruby-versions
You'll have to upgrade your application to at least Ruby 2.4.5 to get it working there.