  Non-magnetic impurities in iron-based superconductors can provide an
important tool to understand the pair symmetry and they can influence
significantly the transport and the superconducting behaviour. Here, we present
a study of the role of strong impurity potential in the Fe plane, induced by Cu
substitution, on the electronic and superconducting properties of single
crystals of FeSe. The addition of Cu quickly suppresses both the nematic and
superconducting states, and increases the residual resistivity due to enhanced
impurity scattering. Using magnetotransport data up to 35 T for a small amount
of Cu impurity, we detect a significant reduction in the mobility of the charge
carriers by a factor of ~3. While the electronic conduction is strongly
disrupted by Cu substitution, we identify additional signatures of anisotropic
scattering which manifest in linear resistivity at low temperatures and
$H^{1.6}$ dependence of magnetoresistance. The suppression of superconductivity
by Cu substitution is consistent with a sign-changing $s_{\pm}$ order
parameter. Additionally, in the presence of compressive strain, the
superconductivity is enhanced, similar to FeSe.
