OpenMPI-bin error after update (K)Ubuntu 18.04 to 20.04

First remove the update-alternatives for openmpi:

sudo rm -f /etc/alternatives/mpi* /var/lib/dpkg/alternatives/mpi*

Then install openmpi-bin again:

sudo apt install open-mpi

answer Y when asked for confirmation.

In Ubuntu 22.04 (or if the above line doesn’t work) – try this instead:

sudo apt install openmpi-bin