Hi @Roger Roger
Welcome to our forum!
What is your user's account? (Exchange/POP/IMAP)
As i know, this feature is not available on IMAP or POP accounts such as Gmail, Yahoo, or iCloud.
If your user is using a M365 account and the button is still greyed out, it is suggested to restart Outlook or even the computer, and then please refer to the article: Schedule send for Outlook on Mac - Microsoft Support to see if that resolves the issue.
If all above do not work, considering data file corruption, it is proposed that the user creates a new profile, re-adds the account to the new profile and checks if the same issue still occurs.
Hope it helps!
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