With harmon.ie, you cannot access personal OneDrive/My Sites that are configured with a custom host.
Your M365 administrator configured the SharePoint OneDrive/My Sites with a different host than the other SharePoint sites. For example, the SharePoint site host is domain.com, and the OneDrive/My Site host is mysite.domain.com. harmon.ie cannot detect the OneDrive/My Site host setting and uses the standard SharePoint host URL.
Specify a mapping between SharePoint host URLs and OneDrive/My Site host URLs.
For each mapping, add a new String Value to the MySiteHostMapping registry key. To learn how to do that, read harmon.ie for Outlook - Administrator's Settings.