In certain scenarios and depending on the Outlook client you are using, you may either not have the option to report a message as spam/phish or receive an error when you do so.
You are attempting to submit a message as spam/phish when interacting on behalf of another account/mailbox you have access to.
The "Report Spam" or "Report Phish" buttons will not appear when viewing a message.
When you attempt to use the "Report Spam" or "Report Phish" buttons, you receive the following error - "You cannot perform this action. Add-ins are not supported in this folder."
Microsoft has confirmed that add-ins only work on the default account you are logged in with (authenticated). If you would like to have this feature enabled for shared accounts/mailboxes, please review the following user voice thread.
If you want to submit a message as spam or phish, please follow the instructions in the Office 365 - Submit a message as spam/phishing document within the "Outlook on the web (do not use Outlook desktop client)" section.