I had the same problem. Finally found two simple solutions.
Solution one
Goto official python site (h**ps://www.python.org/downloads/release/python-390/) (OpenPli 9.0 uses python version 3.9) download tarball,
extract missing dataclasses.py and copy it to /usr/lib/python3.9/ on your box.
Solutions two (recommended)
Install a package which contains the missing dataclasses.pyc:
> opkg install python-3-profile
Thats all. Happy yt-dlp - ing!