DOOM Eternal Fortress of Doom: Secret About Crucible will taunt you until you can access it towards the end of the game. There will be some spoilers, so be warned.
The Crucible secret is but one of the many hidden collectibles in DOOM Eternal. In the Fortress of Doom, there is a question mark on the map above the Crucible. You won't be able to reach it until late in the game, right before it ends.
DOOM Eternal Fortress of Doom: Secret About Crucible
The earliest point you can access the secret is after you get the sword and Samuel Hayden. Look to the top right of the room, and you'll see a red switch. Shoot at it, and a launch pad will appear to allow you access to the secret.
The secret is a cheat code: Fully Upgraded Suit. This will unlock every upgrade and perk for your Praetor suit. This is why you have to wait so late in the game to get it.
Once you're literally fully kitted out, you can take on the end of the game and feel invincible.