Attempting to find total drag force on an object

I'm fairly new to COMSOL and haven't been able to find any really detailed guides on how to go about this. I've currently got two problems that I can't figure out how to solve.

1) When I solve the simulation, I get an error reading:

"Cannot evaluate expression.
Failed to evaluate variable.
- Variable: mod1.fsi.mises
- Geometry: 1
- Boundary: 22
Failed to evaluate expression.
- Expression: mod1.fsi.mises
- Plot: surf1 (Surface)"

Boundary 22 is my inlet flow. I'm not sure how big of a deal this is because I don't believe I need the von mises stress for what I'm trying to solve for. What I'm trying to figure out is how to get...

2) The reaction forces. How can I find the forces on the solid part of my model? And how do I go about integrating them to find the total drag force?

Thanks a ton