External user support is not supported for nextcloud15. How to update it manually
They moved the external users app to a dedicated app, it’s not shipped by default. Normally, they want to push it to the app store soon, but you can install it manually:
(just download the everything as an archive and extract it to your apps/-folder. The folder name of the extracted folder must be the same as the app’s name user_external. It then should show up on your apps page to enable.
Thank you. Your method solved my problem