Maybe it's because you're not a member of the Plone Developers team. See the contributing documentation.
then how can i contribute , if I am not able assign myself work
Hello,
Please take a look at Contributor's agreement and how to be part of the Plone Core Developers team.
Cheers,
Érico Andrei
then how can I be part of it as I want to contribute in Plone for GSOC 2023