When selecting Force Reaction Probe, there are several options for location, as shown below;

The options, in order, are 

• Boundary Condition
• Contact Region
• Remote Points
• Beam
• Spring
• Mesh Connection
• Surface

In order to include a reaction probe for a Contact region, you must first add an Output Control, under Analysis Settings back up under Static Structural. Set 'Nodal Forces' to 'Yes', as shown below;

Then you must RE-SOLVE THE SOLUTION, or the new output control that you included will not be taken into effect. Delete the probe, and rerun. Once complete, you may re-insert the reaction probe and select Contact.