can't install deb package for beta2 on debian 12

The common package installs fine but when I want to gdebi siril_1.4.0~beta2 it says dependency is not satisfiable: libavcodec61. I have libavcodec59 installed, but 61 is not in the repositories. If I want to install the downloaded dep package for it, it needs more dependencies that are not met, and so on.
I’ve searched about that problem but haven’t found anything useful.
Flatpak install works fine but I can’t make use of the python scripts.
Any help is appreciated, thank you.

This is because the package is for debian testing or unstable. Not for stable.

Thanks, adding testing repository worked.