As part of the campaign in Pokemon Legends: Z-A, you will get to head into the Lumiose City Sewers in order to find a missing person alongside Emma. However, the “entrance” you use to get in for that mission is a one-time use one only. In order to get back into the sewers after getting to Rank D, you need to take on a side mission quest. And, since there are exclusive wild Pokemon underground, you will want to go back in again in the game.
How do I get back into the Lumiose City sewers in Pokemon Legends: Z-A?
The key to getting back into the sewers is to wait for Side Mission 57 to show up. This is called “The Camerupt Entrepreneur.” You’ll want to look for Herup in the Bleu Sector. The blue exclamation point icon will show up around the Pokemon Center near Cafe Woof and Cafe Classe. (Both appear on the borderline street between Bleu Sector 3 and Vert Sector 4. After talking with Herup and his goals underground, he’ll tell you to meet him at the entrance to head into the sewers.
Now, to actually reach that entrance may look a bit vague. So here’s a map showing how to walk to where it is:

Once you meet with Herup there, he’ll unlock this door to the Lumiose City sewers permanently. Head down. Meet him in the designated room on the map. Battle his Camerupt. (Don’t worry, as it won’t Mega Evolve.) If you chose Totodile, your evolved Feraligatr will be a perfect opponent. After you beat him, you get 4,500 coins, a Fire Stone, and the Charcoal held item.
From here on out, you can use that entrance to go underground at any time. A permanent map icon will also appear there too, to act as a constant reminder of where the new entrance to the sewers is.
Here’s how that icon looks:

Pokemon Legends: Z-A is available for the Switch and Switch 2.