Using Outlook for iOS for personal email and calendar management
Hi,
I understand that you wanted to remove folders from your Outlook App on your mobile phone. Unfortunately, deleting folders is currently not supported in the Outlook app for iOS and Android. To manage or remove folders, please sign in to Outlook Live on your web browser. I recommend using a desktop browser. If you want to use the mobile browser, make sure it is in desktop mode since it will prompt you to use the Outlook app for IOS.
Source reference: Can I delete a folder on Outlook?
If you require further assistance, please don't hesitate to let me know.
Stay Safe!