calculate granular stress tensor in a specific region with fix ave/euler command

This cannot be done with the current LIGGGHTS implementation. You need to modify the code such that it accepts a different region. The easiest way to go would be to use an equally divided box region. In that case you can easily exclude particles outside the box from the stress calculations and also identify the cell a particle is located in. If you go for irregular region/cell shapes then this becomes more complicated and also the cell search will take up quite some time if you use a brute force approach (I ended up with a hex-mesh based region and a binary search tree for the cell search).