Alternate means of installing Mullvad Browser
Although you have to install the Flatpak itself first.
Then go to Settings/Applications
Then just under the center.
Refresh Applications
After awhile. Mullvad Browser appears in left hand list
use “>” to put it on the other side.
Apply / OK
You should now have a Tab to start “Mullvad Browser”
And for all you effort to install Flatpak, you have all those other easier to install programs shown in Flathub.