thanks @folbert, I think that I had composer 2 in the first instance, but I downgraded again to 1 because it failed to resolve dependencies because I guess some plugins still rely on composer 1.
in the meantime, I finalised the website with composer 1, so I am ready to push it live (it needs to be live on Wednesday so I am running out of time, I am stuck here since 2 days 
).
I can give another go with composer 2, but I am scared because as I said it is all done and ready to go live. (I am not sure if I will have enough time to find/replace other plugins that work with composer 2)
@JordanC26 / @Twansparant I saw your topic (Trellis 1.7.0 with Composer 1.x) , and I think that I have the same problem. (sorry if I drag you here, but that post has the comments closed).
did you find a workaround with that?
do you have any suggestion?
thanks