A Team member cannot add an attachment to a Teams Chat to another of her team channel mates. TEAMS!
She can give permission following the instructions in the error but this error happens every time she sends an attachment.
Not that it should matter but she is a MAC user.
>>> We altered the above behavior behavior by changing the SharePoint settings from specific users to internal users
we need to allow external users to be invited to work on SharePoint files/folders
Changing the SharePoint server settings to allow people to attach documents within a Teams chat seems to then make it so that Sharepoint no longer shares to external guests.
Unless I turn on Anyone With the Link which I do not want to do.