In my tenant, a previous admin enabled shared channels in Teams, but failed to fully configure it. This means that when a user tries to invite external users to the Teams-connected SharePoint site, they get this error:

Your organization’s policies don’t allow you to share with these users. Go to External Sharing in the Office 365 admin center to enable it.

You can enable shared channels in the Teams admin center, but for it to work correctly, you need to configure a few extra things in Entra: https://learn.microsoft.com/en-us/previous-versions/microsoft-365/solutions/collaborate-teams-direct-connect?view=o365-worldwide#configure-cross-tenant-access-settings-in-microsoft-entra-external-id

Lesson learned:
Regardless of the SharePoint site settings, you won’t be able to share files, folders, or content with external users until the Entra settings are in place.
The site’s external file sharing settings will not override the Entra settings.
First, adding the user to Entra as a guest does not help.
Adding a guest to content via PowerShell, Python, or MS Graph API does not work.

Useful links:
https://learn.microsoft.com/en-us/MicrosoftTeams/shared-channels
https://learn.microsoft.com/en-us/previous-versions/microsoft-365/solutions/collaborate-teams-direct-connect?view=o365-worldwide