Can't send files to external users

Dear @Chan Narak, 

Thank you for posting your question in the Microsoft Q&A forum.   

1. Could you please clarify, are you unable to send the file directly, or is the attach file option missing entirely? Please provide more details about what you're seeing, and if possible, send a screenshot of your current scenario so I can get a better visual of the issue. 
2. Also, I have one more question: Has this external user been added as a Guest in your tenant yet? 
3. You mentioned that you can't send messages to external users now, were you able to send them normally before this? Also, could you check with a colleague in your organization to see if they have the same issue? If it fails for everyone, your company may have blocked external communication. If it fails for only one person, please attach a screenshot of the error so I can check it for you.

Please help me answer those three questions. In the meantime, I’d like to share some information regarding external sharing: 

External Access - Guest Access: By default, Teams allows you to chat with external users, but direct file sharing in chat is often disabled for security reasons unless that person is added as a Guest in your tenant. 

Reference:  

Use guest access and external access to collaborate with people outside your organization - Microsoft 

Collaborating with people outside your organization | Microsoft Learn 

SharePoint/OneDrive Settings: Since Teams stores files in SharePoint and OneDrive, if the external sharing level there is set to 'Only people in your organization,' Teams cannot override that restriction. 

Reference: Manage sharing settings for SharePoint and OneDrive in Microsoft 365 - SharePoint in Microsoft 365 … 

If you need to send a file right now: You can try uploading the file to OneDrive, selecting it, and clicking Share. Change the settings to 'Anyone with the link' (if allowed), then copy the link and paste it into the Teams chat. You can also try choosing 'Specific people' and entering their email address. (However, please note that if your tenant blocks this in the SharePoint/OneDrive settings, this method will not work either). 

Invite them as a Guest: Ask the Global Admin to add the person as a Guest in Entra Admin Center. Quickstart: Add a guest user and send an invitation - Microsoft Entra External ID | Microsoft Learn 

Please help me share more details about the specific error or behavior you are experiencing so I can assist you more effectively. 

Looking forward to your response! 

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If you are unable to send files to external users in Microsoft Teams, it may be due to your organization's external sharing settings. Specifically, if external sharing is turned off for your SharePoint or OneDrive sites, you will encounter issues when trying to share files with users outside your organization.

To resolve this issue, you may need to check the external sharing settings for your organization and ensure that they allow sharing with external users. This can typically be done by a SharePoint or Microsoft 365 administrator.

Here are some steps to check the external sharing settings:

1. Sign in to the Microsoft 365 admin center as a global or SharePoint admin.
2. Navigate to the SharePoint admin center and check the sharing policies under the Policies section.
3. Ensure that external sharing is enabled for the relevant sites or OneDrive.

If you do not have the necessary permissions to change these settings, please contact your administrator for assistance.