It looks like Joomla OAuth Server plugin wasn’t installed properly.
Please follow below mentioned steps to uninstall the plugin and install it again.
Uninstalling the plugin:
1) In the Joomla Admin console, Goto Extensions –> Manage –> Manage.
2) Search for ‘miniorange’ in the search box
3) Select all the extensions listed and click on uninstall (make sure you only installed Joomla OAuth Server plugin).
4) After uninstalling please install the plugin again.
1) Goto Extensions –> Manage –> Install.
2) Select Upload Package File and choose the Joomla OAuth server plugin file and install.
3) Then Goto Extensions –> Manage –> Manage.
4) Search for ‘miniorange’ in the search box
5) Enable the extensions present in the list.