Thank you for your question and reaching out.
To check exact error please check log files or enable Remote errors.
Please check logs log files on the SSRS server for more detail. They'll be somewhere like: "C:\Program Files (x86)\Microsoft SQL Server\MSRS10_50.DEV\Reporting Services\LogFiles\
Also : Enable Remote errors for a Reporting Services Service Application
For a SharePoint mode report server installed with SQL Server 2012 (11.x) or a newer version of Reporting Services, enable the service application setting Enable remote errors. The setting can be configured for each Reporting Services service application.
In SharePoint Central Administration, click Manage service applications in the Application Management group.
Find your Reporting Services service application and click the name of your service application.
Click System Settings.
Click Enable Remote Errors in the Security section.
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