Veeam ONE will ask for a license file when running in the Free edition for longer than 90 days.
This is due to software design. Veeam ONE requires a license file to run, regardless if it is a free license or a paid license.
Go to http://www.veeam.com/monitor-free-key, and fill out the information to acquire your free license to run Veeam ONE Free edition. Once you have the license file, apply it via Veeam ONE Monitor client > Help > License information or Veeam ONE Reporter Web UI > Configuration > License.
Please be aware that starting from September 2018 downloading updates will require an active contract for the corresponding product.