How did you configure Ventoy before creating the two sticks – or, if you used the command-line – what was the exact command used to create the Ventoy USB?
This may suggest the file system is not recognised.
Did you configure Ventoy to use exfat (a good choice) for the storage partition? If so, is the exfat-utils package installed?
sudo pacman -S exfat-utils
Assuming the exfat-utils package is installed, the file system is exfat, and both the partition node/number and mount point are correct, try:
sudo mount -t exfat /dev/sda1 /run/media/usb
On the chance that you inadvertantly borked both USB sticks – perhaps by removing them too soon (example) from the USB port, you can check them for consistency;
sudo fsck.exfat /dev/sda1
Important: The file system must not be mounted prior to using fsck.
To address this…
The output you pasted shows that you are trying to boot an ISO from a network location, which you are not, so it’s telling you the attempt failed. This error would be expected.
You likely need to select the USB manually via F8 or whichever key on your mainboard is required to enter the boot selection menu (not the GRUB menu).
Either that, or temporarily promote the boot order of the USB to before the PXE boot – there will likely be a setting for that in the BIOS.
Please provide your system information as described (below) so that others wishing to help can more easily do so. 
This may be a long post with the welcome template added, but it’s nonetheless important to read through all of it, and follow any links given.
Regards.
What follows is from a standard template.
Welcome to the Manjaro community
As a new or infrequent forum user, please take some time to familiarise yourself with Forum requirements, and the many ways to use the forum to your benefit.
To that end, links are provided (below) - Please use them.
Be prepared to provide outputs from various commands when asked. It’s equally important to provide as much actionable information as possible in your first post, rather than simply indicating there is a problem.
Waiting for others to blindly ask questions can be counter-productive – typically, nobody has a 
at their disposal – Instead, please help others to make informed suggestions, based on information you provide.
Update Announcements
The Update Announcements contain important information and a Known Issues and Solutions section that should generally be checked before posting a request for support.
System Information
While information from *-fetch type apps might be fine for someone wishing to buy your computer, for Support purposes it’s better to ask your system directly;
Output of the inxi command (with appropriate parameters, and formatted according to forum guidelines) will generate information useful for those wishing to help:
inxi --filter --verbosity=8
or the short form (preferred):
inxi -zv8
- inxi man pages (Arch Manual)