To the title issue:
Please review this:
Particularly the bits about checking existing boot entries.
It would appear you have not formatted the existing ESP. Could it be that it is in 32-bit or encrypted?
chroot is for accessing an already installed system from the live media.
If there is no manjaro install on the disk, I’m not sure how that would work …
On to some other things…
This likely will not work well. Calamares itself may struggle with such low memory.
(I would also very much warn you to use some form of SWAP on the resulting installation … as well as ask you to reconsider GNOME on such low memory)
If I am not mistaken this may be able to be remedied by creating some temporary SWAP for the live system to use during installation.
(or you can try an ISO with swap or zram already configured)