When performing a measurement in the HTML5 viewer and the measurement lines appear but no label appears containing the value the issue is most likely due to the geometry service being used.
In Essentials Manager visit the 'GIS Services' page and make sure there is a geometry service set. If you are using ArcGIS Server you can create one if required, please refer to the ESRI documentation for instructions on how to start it (ESRI documentation here).
If you are using a valid geometry service and the labels are still not appearing, run the html inspection tool Fiddler and perform a measurement. You will see a call to the geometry service and the response should contain the values that are to be displayed. Instructions for using Fiddler can be found in our article: Use Fiddler to capture web traffic between a web browser and a web site
If you are still not able to find the cause of the issue, please open a Support ticket.