The HTML5-based report viewers require Kendo UI for jQuery. For some reason the viewer is unable to get Kendo from the Reporting REST service, so you will need to reference it inside your page. A Kendo subset is available inside the Telerik Reporting installation folder, for example, C:\Program Files (x86)\Progress\Telerik Reporting R3 2019\Html5\ReportViewer\js\telerikReportViewer.kendo-184.108.40.2064.min.js.
I noticed that the viewer tries to connect to a Report Server instance. Note that the viewer and the Report Server should be of the same version in order for everything to run smoothly.
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