how can i change the ppi on layer scalling

You can’t change it there.
You must go to Image>Scaling and change both ‘X Resolution’ and ‘Y Resolution’.

See Image > Print size. Also, read this: Image size in Gimp.

i don’t want to change the hole image i just want the layer

Can’t change the ppi of one layer only because the ppi applies equally to all the layers in the image.

May I ask what the purpose is?