Hello Lindsay_1234,
Thank you for reaching out in Apple Support Communities. We understand you're unable to access the website for Outlook on your Mac, due to an error message. We'll be happy to help.
You may need to change the trust settings of the certificate via Keychain Access on your Mac. This may allow you to access the website for Outlook. Here's how to perform this step:
Change the trust settings of a certificate in Keychain Access on Mac
You can view or change the trust policy of a certificate in Keychain Access.
1.In the Keychain Access app
on your Mac, select a keychain from one of the keychains lists, then double-click a certificate.
2.Next to Trust, click the arrow
to display the trust policies for the certificate.
3.To override the trust policies, choose new trust settings from the pop-up menus.
This information is outlined in this helpful article: Change the trust settings of a certificate in Keychain Access on Mac - Apple Support
If this continues, be aware that the connection is just not private, but you can still access the site using "Show Details", and follow the information here: If you see a "Not Secure" warning while browsing the web with Safari - Apple Support
Please let us know if that helps.
Kind regards.