If you would like to use a report from the Report server as a report source of the SubReport item, you need to specify it as follows:
For example Samples/Sales Order Details.
You can also check the initially added reports when you install the Report Server. Sales Order Details contains a subreport. When you download it you can see how the report source is set. Please, take a look at the attached screenshot for additional reference.
Please, test and let me know if you need further assistance.
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