"The thing about this is that I'm not using postgres, so I'm confused why it is saying this."
You're using Heroku, so you are using Postgresql.
In your app.rb you have this line:
DataMapper::setup(:default, ENV['DATABASE_URL'] || "sqlite3://#{Dir.pwd}/notes.db")
which I'm guessing you got from the Heroku database docs. This line basically says "check for the environment variable 'DATABASE_URL', and if it's set use it as the database url, otherwise use the Sqlite url". Running locally this environment variable won't be set, so you'll use the Sqlite url, but on Heroku this will be set to something like (see the page linked above):
postgres://username:password@hostname/database
Datamapper will see that this is a Postgresql url, and try to require the postgres adapter, which isn't installed so will result in the error that you see.
The best solution would be to install Postgresql locally, so that your development and production environments are as similar as possible. If you can't do this, or don't want to, you can specify the Sqlite adapter locally, and the Postgres adapter in production. It'll look something like this in your Gemfile:
group :development do
gem 'dm-sqlite-adapter'
end
group :production do
gem 'dm-postgres-adapter'
end
If you do this you'll need to tell Heroku to which groups to leave out when installing gems, see the Heroku Gem Bunder docs.
