Going to need much more context.

basically in the backend there is a process where when a new user is registered, an opt email is sent to the user with a code. so far this is not working and what I am getting is this

Email sending failed with an error: Error: Missing credentials for "PLAIN"

Jun 24 12:09:58

at SMTPConnection._formatError (/app/node_modules/nodemailer/lib/smtp-connection/index.js:798:19)

Jun 24 12:09:58

at SMTPConnection.login (/app/node_modules/nodemailer/lib/smtp-connection/index.js:452:38)

Jun 24 12:09:58

at /app/node_modules/nodemailer/lib/smtp-transport/index.js:272:32

This would be an issue with your application, not the Railway platform. Unfortunately we're unable to provide help for application-specific issues -- I'd recommend checking out other communities such as https://stackoverflow.com/ for general programming-related questions.