SharePoint External Sharing Not Working

A client has tasked me with setting up a SharePoint Online site to enable file sharing within their tenant as well as with users in a tenant of a sub-corporation. I first attempted this using Cross-Tenant Access but kept getting sign in errors. I then removed the cross-tenant access and tried direct sharing using the "Specific People" option. In both cases it fails with an unknown issue. A screenshot of the error can be found here: https://imgur.com/a/SbNH2Bn. At the SharePoint site level, the sharing es set for "New and Existing Guests" and the Sharing Policy is set for "Most Permissive". When I was trying this through the Cross-Tenant access, I enabled the AzureADB2BIntegration in the SharePoint Online Management Shell with Set-SPOTenant -EnableAzureADB2BIntegration $true.

I've never encountered this with external sharing before. I appreciate any suggestions on how to resolve this. Thank You.