I also had the same issue when I configured cron on an AWS EC2 server. This was allowed by setting the package path explicitly in the config/schedule.rb file.
set :bundle_command, "/usr/local/bin/bundle exec"
This creates an entry in cron, like:
30 1 * * * /bin/bash -l -c 'cd <app_path> && RAILS_ENV=beta /usr/local/bin/bundle exec rake 'task_name' --silent >> log/cron.log 2>&1'
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