Du you have sudo istalled? repetierserver has no password so you can not use it to login directly.
On installation it adds a file
/etc/sudoers.d/repetierserver-perms
that allows running klipper install as root due to these lines:
repetierserver ALL=NOPASSWD: /usr/local/Repetier-Server/bin/installInputShaping
repetierserver ALL=NOPASSWD: /usr/local/Repetier-Server/bin/createFirmware
repetierserver ALL=NOPASSWD: /usr/local/Repetier-Server/bin/installKlipper
repetierserver ALL=NOPASSWD: /usr/local/Repetier-Server/bin/uninstallKlipper
repetierserver ALL=NOPASSWD: /usr/local/Repetier-Server/bin/restartKlipper
repetierserver ALL=NOPASSWD: /usr/local/Repetier-Server/bin/inputShaping
repetierserver ALL=NOPASSWD: /usr/local/Repetier-Server/bin/updateKlipper
repetierserver ALL=NOPASSWD: /usr/local/Repetier-Server/bin/getKlipperLog
repetierserver ALL=NOPASSWD: /usr/local/Repetier-Server/bin/installRepetierNightly
repetierserver ALL=NOPASSWD: /usr/local/Repetier-Server/bin/installRepetierLatest
but this only works when sudo is installed. When you get asked a password it would normally mean sudo is installed, but the file is missing or being ignored. We assume that the starting /etc/sudoers file contains this line:
@includedir /etc/sudoers.d