Hello, I ve been started to render using karma and when i export crypto render passes I cant access them in nuke or Fusion
Does anyone know an workaround or happen to know how to fix it ?

I've seen older versions of Nuke won't read the cryptomatte built into the main EXR. Probably similar issue with Fusion.
You can use the cryptomatte LOP and export as a separate pass instead. I believe keeping them as separated files is generally recommended.

With Nuke 14 I've been able to read and use the Cryptomattes baked in the main EXR though.

try checking Enable Legacy EXR Mode on USD Render ROP or Karma ROP