Particle tracing - particle counting

Hello to everyone,

Im new to COMSOL and currently trying to work with particle tracing for fluid flow.
The model we are working on is part of vascular network and we are using as physics : structural mechanics, laminar flow, darcy flow, and particle tracing for fluid flow. Everything is working up to the point where we can calculate the particles trajectories but we need to be able to calculate how many particles pass though a specific area section. I have tried to use the Total number of particles in selection, Particle density, Particle position from the results in order to accomplish this. Unfortunately when i try this i get the following error:

CANNOT EVALUATE EXPRESSION
failed to evaluate variable -
Geometry -
Domain -

I have tried to toggle " compute particle mass" on from the particle tracing menu but then when i do this and try to calculate the solution i get another error:

Solution cannot be updated. Clear Solution to expunge.
Failed to evaluate expression.
Failed to evaluate variable.
- Variable: mod1.mp
- Geometry: 2
- Boundary: 1
Failed to evaluate expression.
- Expression: mod1.mp


I have checked the example of laminar_mixer_particle and the model but at that particulate example it only calculates trajectories and nothing more.
I've also tried to export the particle coordinates (designated as q) to excel spreadsheet file so i can process them elsewhere but been also unsuccesfull at that.

If someone has a solution please respond
Thank you,

SM