If you just installed x-pack then you currently have a trial license which will expire in 30 days. To continue to use Monitoring and Reporting you would need to register and get a Basic license (free).
Then you would install that license in your cluster (to any node since it's a cluster-wide thing). Here's some instructions;
https://www.elastic.co/guide/en/x-pack/current/installing-license.html
Regards,
Lee