Substitution for illegal variable in if structure

LAMMPS LAMMPS Beginners
1

Hello everyone, I get a problem when I tried to use an if structure. My code is below:
#########
label forloopp
variable a loop 50
variable force equal 1.66e-3*v_a
fix increase first addforce v_force 0 0
variable y atom fx[1]
run 1000
if “${y}>5e-12” then “jump SELF forloopp”
next a
jump in.liangbaibu forloopp
#########
It is an overdamped process.My purpose is to make the force in x direction of the first atom as an index. When this force is larger than 5e-12, then I run this system 1000 steps. However, I always get the error message: Substitution for illegal variable in if structure.
I have checked this topic: Illegal Variable Substitution - LAMMPS / LAMMPS Mailing List Mirror - Materials Science Community Discourse (matsci.org). It seemd that it can not address my problem. Thanks for reading and any help will be great.

2

You must not use an atom style variable in an “if” construct. The condition must be some global expression.
Similarly, you must not use an atom style variable in a ${...} style variable expansion.

3

Thanks for your reply!