Running adjoin with a seemingly valid computer name on the command line produces an error about illegal characters:
#sudo adjoin --user Admin --password Pass --name computer-123 --force example.com
The computer name computer-123 is not valid. It may contain illegal characters.
Standard computer names contain letters, numbers, and hypens (-) only.
Spaces, periods and the characters \/:*?"<>|. are not allowed.
Other characters are legal but may lead to interoperability issues.
Computer names may not consist entirely of digits.
It is caused by extra spaces accidentally entered at either the beginning or the end of the computer name.
Run the same adjoin command again after removing all extraneous white spaces.