No, you can’t share a Facebook post from a private group and that’s NORMAL.
If you are on Facebook, you are probably part of at least one “private” group. So you regularly get “Can’t share” comments on a post in that group. It’s NORMAL not to be able to share, and here’s why.
A private group is a group that requires approval to be a “member”. And you have to be a member to post something or, in some cases, to see its content.
So, the principle of a private group is that its publications are not visible to everyone. It is therefore normal that the possibility to share a publication does not exist! If it were allowed, any member of the group could share publicly a publication of a private group, and that is (precisely…) not the goal.
The author has nothing to do with it, he can’t do anything about it.