Error decoding first MiB of image. Caused by failed to fill whole buffer

ignition file:
error when provisioning: Imgur: The magic of the Internet
image used: fedora-coreos-33.20210301.3.1-live.x86_64.iso
Any idea what is causing this error?

Your arguments to coreos-installer are wrong.

sudo coreos-installer install /dev/sda -f example.ign ....

-f is not for the ignition file. Here is a snippet from the coreos-installer install --help:

    -s, --stream <name>             Fedora CoreOS stream
    -u, --image-url <URL>           Manually specify the image URL
    -f, --image-file <path>         Manually specify a local image file
    -i, --ignition-file <path>      Embed an Ignition config from a file
    -I, --ignition-url <URL>        Embed an Ignition config from a URL

lol they are one under the other …