Hi,
First thank you @piero and @aibd for your replies.
This is what I tried already :
- Pressing cancel → see below
- Type my phone lockscreen number (not SIM card PIN) → wrong password
I am offered with a variety of options which I know nothing about.
I went through the options and already try reboot but I always come back to that TWRP screen.
The menu looks like the screen shots in that post : [HOWTO] install /e/ on a Samsung device from a GNU/Linux PC successfully
What should I do?
Install?
Wipe? 
Backup?
Mount?
Honestly I am very afraid to do the wrong stuff and get my phone unusable. 
Thank you in advance
Best regards
Jeff