  We use neutron scattering and bulk property measurements to determine the
single-ion crystal-field Hamiltonians of delafossites $\rm KErSe_2$ and $\rm
CsErSe_2$. These two systems contains planar equilateral triangular Er lattices
arranged in two stacking variants: rhombohedral (for K) or hexagonal (Cs).
  Our analysis shows that regardless the stacking order both compound exhibit
an easy-plane ground state doublet with large $J_z=1/2$ terms and the potential
for significant quantum effects, making them candidates for quantum spin liquid
or other exotic ground states.
