One wall is darker than the other

Development Programming & Scripting Blueprint
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1

Hello!
One wall is darker than the others.

Capture d’écran 2021-12-29 153310
Capture d’écran 2021-12-29 153310617×557 289 KB

Capture d’écran 2021-12-29 153258
Capture d’écran 2021-12-29 153258943×514 19.8 KB

I guess it’s because there’s a “hole” in the mesh but I’m not sure.
How can I fix it?