Ubuntu 20.04 isnt released yet, if you want to test it, please use the official way over compiling the own deb packages. You can even run the installer from master, followed by a v-update-sys-hestia-git master and you would basicly get a working system.
Infact this is not the target at all - because you would also have to include the latest changes for filemanager in combination with hestia-php.
tl;dr: Just use the proposed way of installation for the testing build - Hestia deb packages