Not enough processor slots to run the simulation

Greeting,

I am very new to CFDEM and OpenFOAM in general and just about looking to simulate a fluidized bed combustor.

I looked around in this forum and found that ErgunTestMPI is a fluidised bed, so i wanted to run this file and start modifying it to fit my model. However, I promptly ran into the following problem as I attempted to run the simulation:

--------------------------------------------------------------------------
There are not enough slots available in the system to satisfy the 4 slots
that were requested by the application:
/home/ejm94/LIGGGHTS/LIGGGHTS-PUBLIC/src/lmp_auto

Either request fewer slots for your application, or make more slots available
for use.
--------------------------------------------------------------------------

Similarly,

--------------------------------------------------------------------------
There are not enough slots available in the system to satisfy the 4 slots
that were requested by the application:
cfdemSolverPiso

Either request fewer slots for your application, or make more slots available
for use.
--------------------------------------------------------------------------

This is very strange as when I ran nproc to check the number of processors in my laptop I got 4 as the output, meaning there should be enough processors to satisfy the simulation requirement?

Could anyone please advise me on how to fix this problem? I tried looking for a way to change processor requirement for a solver but I did not find any info on that.

Best,
Napattorn