This page of our guide to Wolfenstein: Youngblood answers the question of how to unlock access to the shooting range in the catacombs' - hideout of the resistance. One of the side missions of the game is connected with it.
You can find the shooting range in the upper level of the Resistance Hiding Place and some neon signs with rifles lead to it. When you get there, you'll determine that the shooting range is closed. However, this is not a useless place, because from the very beginning of the game you will find free armor and ammunition reserves in this spot.
In order to unlock the shooting range you have to take on a side task called Claude the Armorer. You can get it from Anais Fontaine after you have advanced sufficiently in side missions.
The task is to find the title armorer and visit the underground bunker in the Little Berlin district. Detailed description of Claude the Armorer side mission can be found in our walkthrough.
After saving Claude and returning to the catacombs, you will notice that the shooting range is open. You can practice shooting at ordinary Nazis and Supersoldats (a great opportunity to test how well your weapons can handle destroying enemy shields).