i get this error truing to install
Create a new partition (465.46 GiB, ext4) on ‘/dev/sda’
Job: Create new partition on device ‘/dev/sda’
Command: sfdisk --force --append /dev/sda
Command: sfdisk --list --output Device,UUID /dev/sda
Job: Set the label on partition ‘/dev/sda2’ to “root”
Command: sfdisk --part-label /dev/sda 2 root
Job: Create file system ‘ext4’ on partition ‘/dev/sda2’
Command: mkfs.ext4 -qF /dev/sda2
Welcome to the forum!
Um, where’s the error? All you’ve pasted is just regular information regarding what it’s doing.
The error is real - it is caused by Plasma settings → Removable Storage → Removable Devices → Uncheck On Login and On Attach
I banged my head at it 5-6 times before I realized what was going on - a test system to locate another inconsistency in Plasma (Does Fall Apart effect work in your Kde Plasma?).
I saw another topic with the same issue
The error may also indicate the partition is mounted and the installer cannot format the partition.
This issue was known about and should have been fixed in Calamares, but the same is true for a few other issues. Apparently our current install images are not using the latest Calamares yet. 
I am facing the same issue. This is not a good start, a crippling bug at the very first step of the installation.
So what’s the workaround? How are people installing Manjaro??