I read the documentation a little bit and I found out I could add other *owners* into the Firebase account. I added my other account there.
My App as two versions, one for IOS, and on for Android.
Now, when I click to link my admob-app to firebase, it correctly *founds* the firebase-project.
I could link my admob IOS app to the firebase project.
But, could *not* link my admob *Android* app to the firebase project.
The error message is : "Unable to link app to Firebase. Please refresh the page and try again."
(The difference is, prior to finding the firebase-project, I tried, wrongly, to create a new one for the android version. Then I delete it... ).
Thanks for you help,
Cheers,