@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.