If statement

I am trying to use a conditional statement to add forces in my simulation box. I am having difficulty implementing the if statements with the fix addforce command. For example, I would like my particle when found between -6.5 and -5 z coordinate to have a force of -20 in the z coordinate
Using this if statement is not working for me
if “-6.5<z<-5” then " fix 2 addforce 0 0 -20" else "fix 3 addforce 0 0 -0.1

It doesn’t work because you are just making up a syntax and semantics that are not programmed and not documented.

One possibility would be to create a dynamic group containing only the atoms within a given region, and to apply the force to the atoms of the group.

Dynamic groups carry a lot of overhead. A coordinate dependent force is most easily implemented with an atom style variable. For example like this:

variable myforce atom ((z>-6.5)*(z<-5)*-19.9)-0.1
fix 2 mygroup addforce 0.0 0.0 v_myforce

am guessing the variable myforce is an equal variable?

No, my explanation said “atom style”. Nothing else would make sense. Coordinates are a per-atom and not a global property.