If you enter a Google Tag Manager id in the sitemgr, and it is not saving, and your eDirectory system was upgraded from a prior system, the cause may be missing key value pairs in the Google_Settings table.   This was a known bug in the upgrade script that has been resolved.

If you have a source version, you can add these 2 rows to the Google_Settings table.  After they are added it will allow the key to be saved.

google_tag_status too,  ""
google_tag_client, ""

If you have a hosted version, please contact eDirectory support.