I didn't give up and I found in the log that the problem is with groff-base cheksum.
I checked the groff-base checksum in the iso file and in the installation pen drive. I found that the groff-base checksum in the iso file is correct but in the pen drive faulty.
I was looking for another pen drive and I wrote the iso file to it and then I managed to install zentyal 6.2.
Thanks and Regards