A family of Microsoft word processing software products for creating web, email, and print documents.
Dear Jenny Baker1,
Thank you for posting back.
Based on the information you provided, the bookmark worked correctly in Word document, but when you inserted it into Outlook, it got error.
Please follow this link to troubleshoot whether the issue caused by bookmark: Troubleshoot bookmarks - Microsoft Support
If above can’t work for you, I'm afraid this issue needs to be investigated from the backend side via remote assistance and collect the background logs. As this Forum community has limited resources, the backend support team there has more permission than us, they have more resources to investigate the issue from the backend, they have Escalate channel to escalate your issue to check more and collect log reports for the root cause.
For your reference: Get support - Microsoft 365 admin | Microsoft Learn
Sincerely,
Claire | Microsoft Community Moderator