I ran into this a couple times; after creating the new user, two problems can happen:
1. Error: 500 OOPS: vsftpd: refusing to run with writable root inside chroot()
The solution to this is to make the FTP users home directory unwritable:
chmod a-w /home/testuser
2. Access Denied
This happens when the user’s login is set to a shell that doesn’t actually exist,
such as “/bin/false” or “/sbin/nologin” and can be corrected by adding the new method to /etc/shells,
or allowing the user a valid login shell and disabling ssh access via other means.
usermod -s /bin/bash testuser