This error can occur after a firmware update or if the calibration of the arm has not been performed successfully. Also, the error can be observed only when the foxy fraction collector is controlled via display: the arm does not stop after moving to a position at the front row, but makes jerky movements.
In this case, please go from the main menu to the calibration icon and press many times the upper arrow till the arm stops to move. Reboot now the fraction collector. If the fraction collector now behaves normally, a new calibration can now be done: please follow the relative instruction in the user manual.