I used the search function and came up with this existing thread on this same issue that amazingly happened about 3 weeks ago. CentOS 7 Install: Error populating transaction . My issue is very, very similar however I am using mkisofs instead of genisoimage.
The error I am receiving is
Error populating transaction after 10 retries: failure: tar-1.26-29.el7.x86_64.rpm from anaconda: [Errno 256] No more mirrors to try.
When I read my packaging.log I am receiving the same sort of nonsense that the user in that previous thread is getting which is yum is trying to find my package in /run/install/repo/ when my package is with all the other RPMs in /run/install/repo/Packages . His solution was to add -untranslated-filenames to his genisoimage command. I added the similar -U command to my mkisofs and this made no difference for my error.