Can I sell a handpainted large scale 3D model of a copyrighted 2D concept art? The art in question is https://www.instagram.com/p/CIfsO2ZD7Rj/ . I Think the concept artist, Jean Giraud, is dead.
While better fitted to our friends at law.SE, the general gist is: No.
Art is protected by copyright, and any adaption (derivative work) requires the OK from the right holders per se. Only 70-75 years after the death of the author (or publication for company works), a work enters the public domain and the copyright expires.
There are some exceptions (fair use/fair dealing/...), but media transformation is not one of them.
Giraud died in 2012, his estate or heirs - or whoever he/they sold the commercial rights to - own the right to ok derivative Works till around 2087.