I have tried to grant a group the permission, ., but it doesn't work. Do I have to grant all of the permissions one by one? It is a huge work.

I think you might be thinking of the Astrix * wildcard
It's not recommended to use wildcards for any mod.

Thank you. If I want to grant a group all of the permissions for a plugin such as Nations, can I just grant the group Nations.*?

That's how that would work. yes.

edit:
But again, it's strongly discouraged to use wildcards.
Since you're likely using Sponge, you can also use something like nations.admin or nations.command.nation, nation.command.zone or nations.command.nationworld to grand all permissions in those sections.

Thank you very much! 👍