error message in Abaqus CAE

 MPI launcher '' not found.If an appropriate release of MPI has been installed, use the mp_mpirun_path Abaqus environment variable to specify the correct path. If an appropriate release of MPI is not available, set mp_mode=THREADS in the Abaqus environment file to access the thread-based parallel features. Abaqus recommends using MPI-based parallel execution.

abaqus 6.13 ran without prblems on the same machine.

What to do?