The UUID of your root filesystem is incorrect. If you reformatted the partition formerly used by the Manjaro KDE installation, then your UUID will have changed.
Boot up from the installer medium ─ CD/DVD or USB stick ─ in live mode. Open up a terminal window and issue the following commands… 
sudo su -
manjaro-chroot -a
If offered a choice, select the partition with Manjaro. Then run the following command…
lsblk
… and copy over the UUID of the Manjaro partition. Next, issue the following command…
nano /etc/fstab
Look for the line that mounts your root partition and verify that the UUID is the same as what you’ve just copied. Change it into the latter if necessary.
Save the file with Ctrl+O followed by Return/Enter and exit the editor with Ctrl+X. Now issue the following command…
nano /boot/grub/grub.cfg
Verify that the UUID listed in there for the Manjaro root filesystem also matches the one you just copied, and if not, change it ─ note: there may be multiple instances of the Manjaro root UUID in the file, so make sure they are all correct. If you had to change them, save the file and exit the editor as you did earlier in the case with /etc/fstab.
Now issue the following commands ─ I’m not sure whether the first one is needed, but it won’t hurt…
mkinitcpio -P
sync
exit
It should now be safe to reboot, and if all went well, you will be booting with success. 