Gmx genion -s ions.tpr -o solv_ions.gro -p -pname NA -nname CL -neutral

GROMACS version: gmx genion, version 2020.1-Ubuntu-2020.1-1
GROMACS modification: Yes/No
Here post your question

I have problem when I run this command, there is no the serial number of solution 15 , how can I solve this ?

gmx genion needs as input an index file with the water molecules. This is missing in your case. You can check if your tpr file contains information on water molecules.