You are unable to complete the App Activation and a warning message appears.
"Warning: Different K2 Server Found"
The SharePoint site has already been integrated with a different K2 server. For more information, please refer to the following KB on Integrating K2 for SharePoint with Multiple K2 Servers.
There are still references pointing to the previous K2 server. You may check the K2 Settings list for any fields which continue to point to the old server:
https://[SharePoint Site URL]/lists/K2%20Settings
To resolve this, update the fields
as necessary then re-run the App Activation