If you are not receiving the sign-on code from the citizenM | Booking Hotel Rooms app, follow these steps to troubleshoot and resolve the issue:

1. Check Your Spam/Junk Folder: Sometimes confirmation emails can mistakenly end up in your Spam or Junk folder. Ensure to check these folders in your email account for the sign-on code.

2. Ensure Correct Email Address: Verify that you have entered the correct email address associated with your account. A simple typo can prevent you from receiving the email.

3. Email Server Issues: If you are using a specific email provider, check if there are known issues with their server. Email delivery problems on the server side can affect your ability to receive emails.

4. Resend the Code: If you do not receive the code within a minute, you can click on the "Resend" option in the app to attempt to receive the code again. This might help in case the first email was delayed - learn more.

5. Check for App Updates: Make sure that you have the latest version of the citizenM app installed. Sometimes, functionality issues are resolved with updates - learn more.

6. Contact Customer Support: If you have attempted all of the above and are still not receiving the sign-on code, reach out to citizenM support. You can contact them through the official website or via the contact form - learn more.

7. Try Alternative Login Options: If available, try using alternative methods for signing in, such as using SMS verification if the app provides this option.

By following these steps, you should be able to identify and potentially resolve the issue with not receiving the sign-on code. If problems persist after following these actions, contacting customer support will likely be the most effective next step.