Dear AnonymousHL,
Good day! Thank you for posting to Microsoft Community. We are happy to help you.
Based on your description, I understand that you are unable to open a shared a SharePoint Excel file in desktop app as a gust user.
Even though you have editing access, the other organization’s SharePoint settings might restrict external users from opening files in the desktop app. You can ask the SharePoint admin of the external organization to verify their external sharing settings in SharePoint Admin Center: Go to Microsoft 365 Admin Center > **SharePoint Admin Center >**Check under **Policies > Access control > Unmanaged devices >**Ensure that "Allow full access from desktop app, mobile apps, and the web" is enabled > save and close.
If the scenario above is not consistent with yours, you can also post back and point that out. At the same time, to better understand your situation, could you also provide the information below?
- Are you facing any errors? If so, could you please share a screenshot of the error page?
- Could you please check if the checkout function is enabled for this document? If so, please check out the documents before editing.
- Is this issue only happening to one specific file? What about other files?
- Are you only suffering in this issue? What about others?
If you still have further issues, feel free to post in this community again. I am always here to help you.
I appreciate your patience and understanding and thank you for your time and cooperation. Have a good day.
Best Regards,
Sophia