A Microsoft file hosting and synchronization service.
Hi Sarah,
From your description, we can understand that you’re not able the Share or Grant Access to the External users. If you want to Share or Grant Access to the external user, you need to change your Sharing level permission to anyone.
Refer to this article: Managing sharing OneDrive and SharePoint to change the permission level and check the outcome.
After changing the permission level of OneDrive and still problem persists, please provide the screenshot of the error message encountered to move further.
Thanks,
Chitrahaas