SSD not working / Update to HS impossible

Hello,

I just bought a new SSD Owc for my mac Air 13 A1466 mid2013 but it doesn't show up anywhere.

So now I am running my laptop from an another external drive on Moutain Lion

I've been reading that I have to be on High Sierra to find the SSD OWC but I am not able to update to High Sierra because "you may not install to this volume because the computer is missing a firmware partition"... I tried to re-install everything but nothing is working.

Do you have any advices for me to make the update possible and then find the new SSD I bought ?

Thank you a lot,

Kinds regards,

Yv'