You may also be asked to enter some contact information. You can just pressENTER to each of these.
If you only wantftpuser to log in via FTP, you should disable their SSH access by blacklisting their username in the SSH config file. Otherwise, skip to Step 4.
Open the SSH config in nano.
sudonano /etc/ssh/sshd_config
Add the following to the bottom of the file replacing ftpuser with the user you want to deny SSH and SFTP access. You can add multiple users here separated by a single space. (To paste in nano, press the right mouse button).
/etc/ssh/sshd_config
DenyUsers ftpuser
To save file and exit, press CTRL + X, press Y and then press ENTER.