So I installed gem5 according to documentation : https://www.gem5.org/documentation/general_docs/building
There is an example to run : scons build/X86/gem5.opt -j 4
When I tried that I got this error : bash: /usr/bin/scons: No such file or directory.
When I am in gem5 directory if I type ls I can see these files: 
Do you know what's the problem ?
