"E:failed to mount" on Samsung Galaxy J5 after factory reset I have Samsung Galaxy J5, and after I do a factory reset, I got this error message

E:failed to mount /data (Invalid argument)
can't mount '/data' (Invalid argument)
E:failed to mount /data (Invalid argument)
E:Can't mount /data/log/recovery_log.txt


What does it mean and how to fix it?
your phone may be damaged now, So, I think that You have to install Stock ROM again. your phone will not avoid warranty. Stock Rom is default and official Rom.

But, some cases, do not we want to install Stock ROM again .. If we can force reset Samsung phone. Then it will settle. So you had better try to reset from PC before flashing ..