Hello, Donna Penn.
Welcome to Microsoft community.
According to your description, I understand that you cannot get into Microsoft defender in iPhone . Please don't worry, I will do everything I can to help you.
I would like to confirm more specific information with you so that I can more accurately determine the problem you are encountering and better assist you:
- You can share a screenshot of specific prompts or error codes with me when you can't use Office so I can get a more visual idea of what you're currently experiencing.
- What is the source of your download ? Have you tried to uninstall and reinstall it?
📖 -<Installing Microsoft Defender - Microsoft Support>

( I have taken screenshots of the key steps to make sure you find it correctly)
Feel free to share updates with me on how things are progressing after completing the mentioned steps. Should you have any questions or require clarification, do not hesitate to reach out to me. I‘d like to do my utmost to assist you.
Best Regards,
Hannah.Z- MSFT |Microsoft Community Support Specialist