empty "lsmod | grep loop"

The error message mount: /mnt/uiso: failed to setup loop device for /home/sebas/isos/ubuntu-16.04-server-amd64.iso does not seem to agree with your mount point creation sudo mkdir /mnt/myiso

You should either change the mount point to uiso or fix the mount command to point to /mnt/myiso:

sudo mount /home/sebas/isos/ubuntu-16.04-server-amd64.iso /mnt/myiso -o loop

Seems there was a CRC problem with the ISO file.

I downloaded it again and verified as Ubuntu indicates ...

echo "ea6ccb5b57813908c006f42f7ac8eaa4fc603883a2d07876cf9ed74610ba2f53 *ubuntu-18.04.2-live-server-amd64.iso" | sha256sum --check

... and now it mounts ok.

Strange there was no error message indicating the source of problem. Thanks anyway.