Title says it all. I can’t find a .deb file to download. I can’t get it with $ sudo apt-get install synfigstudio. The .appimage file won’t open. Finally, I can;t find it on the Ubuntu Software Center Thanks for your help.
.deb fils are not available anymore for download.
.appimage have the advantage to be compatible will all the main Linux OS and avoid to maintain .deb and .rpm
Avoid to use a Live USB, even with persistence, such system has limitations and different rights than expected in a normal version. Be aware that Live
I always recommend to install VirtualBox (free and open-source mostly, from Oracle) especially for Windows users.
Straightforward to use, you can install your Ubuntu and access Synfig seamlessly from Windows ^^
You just need to defined some shared folders on both Guest and Host systems, your Synfig files inside will be reachable.
In Ubuntu like in many distro, the packages are maintained by official maintainers and there is not always the last version available, they have to create them from the sources and configure them according each distro specs.
Your live version has to be updated/upgraded first otherwise it has access only to the former content of repositories.
The .appimage will open only after execution permissions are given.I cant open the file
Sometimes access rights on /tmp are also responsible of this appimage
So the problem is not about Synfig, but basic knowledge about Linux 
Hello @b3u! Please check instructions here on how to run appimage - How to run an AppImage - Using AppImages - AppImage