A majority of Backupify/Datto SaaS Protection for Google Apps customers have their Gmail services backed up using the REST API.
A small number of Gmail services are still being backed up using IMAP. If that's the case, this information would apply to you.
The Notification 'Your 'Allmail' label is inaccessible through IMAP. To ensure all mail is backed up, please enable IMAP access to 'Allmail' in your GMail settings' is being displayed because that user has their AllMail label hidden from IMAP in their GMail settings. When we run a Gmail backup, we request the records from the AllMail label in Gmail, if that label is not showing over IMAP then we backup all other labels that are showing over IMAP. As a result of that we display a message on the notifications that makes you aware that there is no AllMail folder visible to us.
Typically this is OK since we will grab all other labels to backup. However, if the user has also hidden the other labels from IMAP then we cannot get those records.
Please log in to Gmail as the user in question and go to their Gmail settings. There is a tab in the settings called "Labels". Locate the AllMail label and select the checkbox to "Show in IMAP". The next backup job will then pull over all records to be backed up from their Gmail account.