Setting up Heroku rails GOOGLE_APPLICATION_CREDENTIALS (firebase) error

I have a rails application that use firebase firestore for database application. after I run these command:

export GOOGLE_APPLICATION_CREDENTIALS="/home/user/Downloads/[FILE_NAME].json"

to set the credential for local usage, the application works fine on local. There are issues when I try to deploy on the heroku server. when I run heroku run rails console, I got this error:

/app/vendor/bundle/ruby/2.3.0/gems/googleauth-0.6.2/lib/googleauth/application_default.rb:61:in `get_application_default': Could not load the default credentials. Browse to (RuntimeError)
https://developers.google.com/accounts/docs/application-default-credentials
for more information

then I go to the heroku dashboard, on config vars, set the var GOOGLE_APPLICATION_CREDENTIALS with empty value first, then in terminal I run this:

heroku config:add GOOGLE_APPLICATION_CREDENTIALS="$(cat file/path/to/credential-file.json)"

I still got error after I run heroku run rails console:

/app/vendor/bundle/ruby/2.3.0/gems/googleauth-0.6.2/lib/googleauth/credentials_loader.rb:83:in `rescue in from_env': Unable to read the credential file specified by GOOGLE_APPLICATION_CREDENTIALS: file { (RuntimeError) } does not exist

Any help? I'm referring from these instruction :

https://cloud.google.com/docs/authentication/production