Then you probably have either a very old mpv or one that was compiled without including the "gpu-next" video output driver.

Or else the GPU drivers for your GPU card have not been updated for a very long time.

A recent mpv and recent GPU drivers should solve this problem, because it was frequently encountered only up to a couple of years ago.

mpv 0.36.0 from July 2023, nvidia 555.58.02 drivers from August 2024 (Fedora 39).

I would try updating to 0.37.0 or 0.38.0, IIRC it only started to work for me after I updated to one of those (I always build mpv from the latest commit myself, so I'm not sure which release version exactly has the crucial fixes).

My output of "mpv --version":

  mpv 0.38.0 Copyright © 2000-2024 mpv/MPlayer/mplayer2 projects
  libplacebo version: v6.338.2
  FFmpeg version: 6.1.1 

I do not remember which was the first version that worked fine. For some time, "gpu-next" was not included by default in the releases, but you had to compile a custom variant to include it.

Pay attention that "libplacebo" is required. If libplacebo is absent, then the Dolby Vision conversion will not work. This could be the reason of your problem, if your Linux distribution does not have libplacebo as an automatic dependence of mpv, or if mpv has been compiled without support for libplacebo.