Thank you for your suggestion but slepc is installed I just checked with conda list
slepc 3.11.2 h1ce4c84_1 conda-forge
slepc4py 3.11.0 py37hc459697_0 conda-forge
By noticing that the error message could not load libslepc.3.12 I I installed slepc 3.12and everything works now.
So the cure is simply to conda install -n fenicsproject -c conda-forge slepc=3.12
replacing fenicsproject by your fenics environement name