Hello G_6084,
Thank you for reaching out to the Microsoft Community. My name is Gunasekar and I'm happy to help.
I understand that you are having trouble creating notebook from an 'Existing Notebook Content in Teams'. Here's how to "Set up a Class Notebook in Teams using existing content".
• First, navigate to your class team and select the Class Notebook tab.
• Select Set up a OneNote Class Notebook > From existing notebook content.
• Preview the section groups that will be included in your Class Notebook before selecting Next. These section groups include a Collaboration Space, Content Library, Teacher-Only Section, and a private space for each student in your class.
• Select + Add content underneath Content Library. This will launch a new window listing your Class Notebooks.
• Choose a notebook to copy sections from, then select Next.
• Choose the notebook sections you want to copy to your Content Library and select Done. Repeat this process to copy sections from multiple notebooks.
• Repeat steps 4-6 to copy content into your Teacher-Only Section. Only you and your co-teachers will be able to view and edit this content.
• When you're finished choosing content to copy, select Next.
• Preview and make any edits to the sections that will appear in each student's private space in their Class Notebooks. There are default sections that you can remove or edit. Select + Add section to add additional sections. For example: Labs.
• When you're finished, select Create to begin creating your Class Notebook with existing content. This may take a moment, so feel free to keep working and check back later.
Here's the related article for your reference:
https://support.microsoft.com/help/0a8c3b96-7784-4323-9931-09747c124e7d
If you encounter error "Something went wrong when creating class Notebook from Existing Notebook content", try clearing the Microsoft Teams cache and restart teams. See if it helps.
a. Close Microsoft Teams completely (make sure it's not running in the background).
b. Press the Windows Key + R to open the Run dialog box.
c. Type %appdata%\Microsoft\Teams and press Enter. This will open the Teams app data folder.
d. Select all the files and folders inside this folder (Ctrl + A), then delete them (you may want to back up important files first).
e. Restart Microsoft Teams and try signing in again.
Related article for your reference: https://learn.microsoft.com/microsoftteams/troubleshoot/teams-administration/clear-teams-cache
If the issue persists, I suggest you first try to create it in Teams Online or Teams desktop then check it on your mobile devices. If none worked, I suggest you raise a support ticket from the Microsoft 365 admin center and report the issue to the related team. The more reports they receive from different customers, the more likely they are to be able to find something that affects common users, do investigation, and fix it soon.
Reference: https://learn.microsoft.com/microsoft-365/admin/get-help-support?view=o365-worldwide
Note: To create a support ticket, you need to have Microsoft 365 admin privileges, and if you're not an admin for your Office 365 organization, contact the admin of creating a new ticket.
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I hope this information helps. If you have any questions, please let me know and I will be happy to help you further.
Best regards,
Gunasekar N