Hi JBSFM,
Your screenshot isn't visible. We suggest you use the button "Insert Image" to upload the picture.

Regarding the external user, we would like to confirm the following information to better understand this issue:
1.Can the external user not use Export to Excel feature in the certain list?
2.If you create a new list in the site, can the user export data correctly?
3.Some browsers or environment settings might prevent users from launching or extract data to local disk. So could you help check if you can or the external user can use another fine external user's device to export data with his account?
4.Please share the error message prompt with us.
The above questions can help us confirm the scope in account, list or user's environment to investigate it.
Regards,
Eli