Greetings,
I suspect you’re running macOS Sierra - this repeated activation issue is a known incompatibility between EndNote X7 and this new operating system. If you’re attempting to activate EndNote with a single-user license product key, you should be able to workaround the problematic behavior by reinstalling with an appropriately-signed installer that we’ve got a link to within the following knowledgebase article:
Please let me know if this helps.