Error: /lib64/libstdc++.so.6: version `GLIBCXX_3.4.20' not found

Solution :
Your gcc is older and you need to upgrade new.

If you are using centos, here is the procedure to upgrade gcc

wget https://ftp.gnu.org/gnu/gcc/gcc-8.1.0/gcc-8.1.0.tar.gz
yum install libmpc-devel mpfr-devel gmp-devel zlib-devel*
./configure --with-system-zlib --disable-multilib --enable-languages=c,c++
make -j 8
make install