How to calculate contact forces in Model?

The contact force measure was actually automatically created.  It was named Interface1_force.

To manually create this, the measure type first selected is Interface, not Force as one might think.

The model you attached has only the two parts and does not contain a load of 20kN.

To create the load maybe a fastener could be used like this.Fastener

Your unit sytem "HOF" is not very friendly for force.  A Newton is kg m / sec^2 so 20KN = 20,000,000,000 mm g / sec^2Units HOF

So when displaying study status (*.rpt file) we get 2E10 which is 20kN

Results in HOF units

You could consider an alternate unit system,  for example mm -g - N -ms to help with the basic units shown in the status file.

The measure can also be graphed in the results viewer like this, and in the result viewer you can choose whatever units you desire.

It is just faster and easier to view in the status file as long as you understand what the units will be (basic units)

Results Graph