I have never managed to make this tool work.
Basic example: a purple layer in the background and a green layer above smaller and not centered. To center it, I select the alignment tool, I check that my layer is active and the buttons of the tool are inoperative.



I tried with these 2 options:


So I have to use the guides or the grid ...

Hi Level_0,
Try before, Layer - Crop to content, click on the object and then apply the alignment tools.

Hi Krikor

"impossible de rogner car le calque actif est dejá rogné just'a son contenu"

Ok, then just click on the object (circle's layer) and then apply the alignment tools.
Relative to Image or Relative to first item

That's exactly it, thank you Krikor

it makes appear a small square to the 4 vertices of the dashed square ...

and yet I searched the internet several times without ever finding this indication.

Y'a pas à chercher... faut juste lire la doc:
https://docs.gimp.org/fr/gimp-tool-align.html

EDIT:
ofnuts replied in French ... (so I did the same)

Before asking a question on any forum, graphic or computer, I always take the time to search, so if I come to ask for help, it's because I did not find or not understand .