If you're interested in the solution, it was because the account needs at least "editor" permissions. https://support.microsoft.com/en-us/office/options-for-sharing-and-delegating-folders-in-outlook-for-mac-480d8054-68ce-4150-ba1e-b9b7f2fc4ce5
Microsoft Graph API list categories for shared mailbox
Thank you for your time and attention. I am interested in locating where to view the categories list for a shared mailbox. I have an app registered, it has permissions MailboxSettings.ReadWrite, and my app is using https://learn.microsoft.com/en-us/graph/api/outlookuser-list-mastercategories?view=graph-rest-1.0&tabs=http as a reference for how view all existing categories, but the response shows the default category list, instead of the ones I see if I click on a message in my outlook client and then click on the "categories" menu button. The end goal is to dynamically manage categories (create/update/remove) associated with messages. I have tested the "update message api" https://learn.microsoft.com/en-us/graph/api/message-update?view=graph-rest-1.0&tabs=http and it will update the categories attribute as expected, however I wish for the categories to have a corresponding color scheme beyond just the default "grey" that gets assigned if a category does not already exist. It is my assumption then, that in order for these categories to have the appropriate color scheme ("preset#"), I need to modify the known categories for the mailbox, which is why I want to list the currently known ones, so I may apply modifications (creating new ones, updating color of existing ones, removing ones that are no longer necessary). I would appreciate a reference point on where I am able to get the necessary education to complete this workflow. Best!