This account cannot be used for Family Sharing

There is nearly nothing to be found regarding the error: "I Cannot Add Family Member - This account cannot be used for Family Sharing" Has anyone have a solution or recommendation? I have tried several times to add one of our family member's Apple ID to our Family Sharing group, but it cannot be added for some reason. Their Apple ID works by itself otherwise. Restarting the devices, signing in and out, and trying on a different device did not help.

1. As the organizer, I select Add Family Member....
2. I then type the Apple ID of the member I want to add.
3. I am asked whether I want to Ask This Person to Enter Password or Send an Invitation.
4. I choose Ask This Person to Enter Password.
5. The password for that Apple ID is entered and I select Next at the top right.
6. I am asked to confirm the account to be used, and select Next at the top right.
7. I am then told that I Cannot Add Family Member - This account cannot be used for Family Sharing.

If, in step 3, I decide to Send an Invitation, I still run into problems.

Upon clicking on the Get Started button in the invitation email, iCloud settings is opened, but I get the error Cannot Connect to iCloud.

It is also noteworthy that on this family member's iCould settings screen, the message Family information is not available is displayed.