@Miguel-Ruiz Fluthub appears to be experiencing some technical difficulties at the moment. It has been reported via Matrix and IRC.
Note that the stable version of Strawberry is several releases behind the 1.0.14 release.
$ flatpak search strawberry Name Description Application ID Version Branch Remotes Strawberry Music Player A music player and collection organizer org.strawberrymusicplayer.strawberry 1.0.0 stable flathub
On Ubuntu you might want to add Jonas Kvinge's PPA for an up-to-date version.