Hi Mike,
To share SharePoint files with external users, we'd like to confirm if you have turned external sharing both globally for SharePoint Online(tenant level) and individual site collection level.
At tenant level: SharePoint admin center>sharing>Sharing outside your organization>select an option for external sharing and click OK on the bottom.
At site collection level: SharePoint admin center>site collections>select a site collection>Sharing>Sharing outside your company>select an option for external sharing and click Save on the bottom.
We suggest you refer to the detailed information about external sharing in this article:
Turn external sharing on or off for SharePoint Online
If you still cannot share SharePoint files with external users, please let us know the following information for troubleshooting:
1.A screenshot about the error message you get.
2.Does the issue happen in a specific SharePoint site? Can you share files with external users in other SharePoint sites?
3.Does the issue happen when you share files with a particular external user or all external users?
4.Please go to SharePoint Admin Center, click Sharing, then capture a screenshot of your sharing settings.
Click site collections, select the site you are using and click Sharing, then capture a screenshot of your Sharing settings.
Regards,
Tina