  A search for a new $Z'$ gauge boson associated with (un)broken B-L symmetry
in the keV-GeV mass range is carried out for the first time using the
missing-energy technique in the NA64 experiment at the CERN SPS. From the
analysis of the data with 3.22e11 electrons on target collected during
2016-2021 runs no signal events were found. This allows to derive new
constraints on the $Z'-e$ coupling strength, which for the mass range $0.3 <
m_{Z'} < 100$ MeV are more stringent compared to those obtained from the
neutrino-electron scattering data.
