In Pokemon Legends Z-A, you have to get a move called Ice Beam to advance main mission Mission 19: Reaching Rank D. This can be confusing because it’s the only objective in the game where you aren’t given a waypoint and must figure it out on your own.
You need certain ice-type Pokemon that know this move.
The easiest solution: catch a Vanillite in Wild Zone 12. This Pokemon will be in the level 34+ range and already knows Ice Beam. Wild Zone 12 is in the south of Lumiose City.
It’s the Pokemon that looks like an ice cream cone:
If Vanillite doesn’t already have Ice Beam in its move set, you can change the moves from your box after catching it, or simply catch another one.
Afterward, head back to the main mission marker and command Vanillite to use the Ice Beam attack on the fountain to freeze it.
Then the fountain freezes and the path to the sewers opens up where the quest will continue.
