https://cdimage.debian.org/cdimage/weekly-live-builds/amd64/iso-hybrid/
The files just aren't there (yet?), website probably got updated prematurely.
In case you want to install testing for personal use (not for testing the ISO), the officially recommended way is to install a minimal stable system, change the sources.list to point to testing, do a dist-upgrade and then install what packages you need.
If you don't need a live image, the regular one is here:
https://cdimage.debian.org/cdimage/weekly-builds/amd64/iso-dvd/