I will summarize what I learned and stumbled every day. If you find any mistakes in the description, please let us know! I'm always helped by other people's articles, so I hope it helps.
Gemfile not found error.
Without moving to the directory of the created application,
bundle exec rake db:create
It happens when you execute a command such as.
Use the cd command to move to the directory of the created app and then execute it.