It’s done and it’s working fine !
So happy. And so proud of me 
Thanks to all for your help and all your post and comments. All of them are really usefull.
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Sorry I read your issue just now, but I can confirm that fastboot and adb in the official debian/ubuntu repositories are known to be bugged since more than a year (at least until the 18.04 version, I didn’t test the newer versions)… So, you need to install them from https://developer.android.com/studio/releases/platform-tools as @AnotherElk said.
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You are welcome. Thanks a lot !
I’m discovering /e/OS, it’s really a good job.
Hope the easy installer will soon take in charge the FP3. I can imagine that many people are stressed with the installation. It was my case. Everything is fine, I’m really happy 
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