  The main aim of this paper is to present accurate energy levels of the ground
[Xe]$4f^{12}$ and first excited [Xe]$4f^{11}5d$ configurations of Er$^{2+}$.
The energy level structure of the Er$^{2+}$ ion was computed using the
multiconfiguration Dirac-Hartree-Fock and relativistic configuration
interaction (RCI) methods, as implemented in the GRASP2018 program package. The
Breit interaction, self-energy and vacuum polarization corrections were
included in the RCI computations. The zero-first-order approach was used in the
computations. Energy levels with the identification in $LS$ coupling for all
(399) states belonging to the [Xe]$4f^{12}$ and [Xe]$4f^{11}5d$ configurations
are presented. Electric dipole (E1) transition data between the levels of these
two configurations are computed. The accuracy of the these data are evaluated
by studying the behaviour of the transition rates as functions of the gauge
parameter as well as by evaluating the cancellation factors. The core electron
correlations were studied using different strategies. Root-mean-square
deviations obtained in this study for states of the ground and excited
configurations from the available experimental or semi-empirical data are 649
cm$^{-1}$, and 747 cm$^{-1}$, respectively.
