Personally I'm kinda forget why or when this problem occured, but this was written inside my sacred black note. So I'll put it here in case this will be a recurring problem.
Basically, if EXT4 partitions are broken by accident and they need to be repaired, we will use fsck command to fix it.
sudo fsck.ext4 -y /dev/sda[number]
Source: https://askubuntu.com/questions/910078/structure-needs-cleaning-error-cannot-mount-partition