Thank you for reporting a bug to Unity.
On macOS, /opt is owned by root and is not writable by regular users. Creating a project there requires elevated permissions, which the Hub does not request by design — so the Hub correctly reports that it is not permitted to create a project at this location. Full Disk Access does not override standard file permissions on root-owned directories.
As a workaround, create a user-owned folder with elevated permissions (e.g., sudo mkdir /opt/MyProjects && sudo chown $(whoami) /opt/MyProjects) and select it as the project location.