Ok, so Luma3DS stopped supporting Dev Firmware on v6.5, so you can either use Luma v6.3 (It is not longer store on github), use an inferior Luma Dev version or using another CFW supporting it.
You can use Puma33DS, a CFW based on Luma v6.3:
https://github.com/rboninsegna/Puma33DS
I have full tested it, but I recommend to use v9 and not v10.