Fedora Silverblue is a immutable desktop operating system variant of Fedora Workstation.
Currently it is not possible to install Portmaster on Fedora Silverblue because the system files are Read-Only.
You can currently install Portmaster by downloading the .rpm and running
curl https://updates.safing.io/latest/linux_amd64/packages/portmaster-installer.rpm --output /tmp/portmaster-installer.rpm
rpm-ostree install -A /tmp/portmaster-installer.rpm
sudo rpm-ostree apply-live
However since the file system is Read-Only it cannot download the remaining resources.
If you've made it work please make a compatibility report on GitHub.