one sl is not registered after a lot of attempts!

What other SONOS products are in the system? Is this ONE SL recently added?

1 Arc+2 era 300+ 1 sub+1 one sl+ 2 roam

This one sl was working normally with a other one sl as stereo 2 years ago 

If the ONE SL has been sitting, unused, not updated for a while, I suggest that you Factory Reset the unit.  Take care as you add the unit to your existing system, don’t setup a new system. Likewise when you bring the second ONE SL online.

Which controller are you using? Resetting the controller might help.

It was used everyday, factory reset and the controller Sonos app on iPhone 14 re installed. I attempted to ad it to my existing system! 

When is the last time it was used?  If it has not been used for two years, your firmware may be too out of date to connect wirelessly.  Connect the unit via Ethernet cable, then try to add.  If it adds, wait for it to update, then you can disconnect the Ethernet cable and de-ploy wirelessly.

Excuse me but I mean it was used everyday last 2 years (I’m frensh speaking 😁) 

I don’t use WiFi all speakers are wired

I don’t think thy have a support office in Morocco, so I would call the one in France. It’s possible there’s some hardware issue going on. 

Je ne pense pas que vous ayez un bureau de soutien au Maroc, alors j'appellerais celui en France. Il est possible qu'il y ait un problème matériel en cours.