Gliss nicht einstellbar

English forum (and wrong subforum, should better be https://musescore.org/en/forum/6), English reply:
a) select the starting note, apply the gliss from the palette
b) select the gliss and in Inspector adjust it's settings, the gliss style

See also https://musescore.org/en/handbook/arpeggios-and-glissandi#glissandi