It sounds like you're experiencing issues with checking out documents from a Microsoft 365 library directly from the document itself, as the checkout option is not visible, and the document opens in read-only mode. However, you're able to check out documents successfully from the library interface itself.
Here are some steps you can take to troubleshoot and potentially resolve this issue: Check User Permissions: Ensure that the user accessing the document has the necessary permissions to check out documents. Users must have at least "Edit" permissions on the document library to check out documents. Document Library Settings: Review the settings of the document library where the document is stored. Ensure that document checkout is enabled in the library settings. You can find this option in the library settings under Versioning settings. Browser Compatibility: Try accessing the document using different web browsers to see if the issue persists. Sometimes, browser compatibility issues can affect the functionality of certain features in Microsoft 365 applications. Ensure that you're using a supported browser and that it's up to date. Clear Browser Cache and Cookies: Clear your browser's cache and cookies to ensure that you're loading the latest version of the SharePoint site and document library. Cached data might sometimes interfere with the proper functioning of SharePoint features. Check-in Any Checked-Out Documents: If there are any documents that are currently checked out by the user or other users, ensure that they are checked in before attempting to check out the document again. Checked-out documents might prevent other users from checking out the same document. Document Versioning: Verify that document versioning is enabled in the document library settings. Document checkout and versioning are closely related features in SharePoint, and versioning must be enabled for checkout to function correctly. Contact Microsoft Support: If the issue persists after trying the above steps, consider reaching out to Microsoft Support for further assistance. They can provide additional troubleshooting steps specific to your environment and help resolve any underlying issues.
We hope these steps help resolve the issue you're experiencing. If you have any further questions or encounter any challenges, please feel free to ask. Gratitude for your patience and understanding as we work to resolve this matter!