This will issue will come when SQL Server has been upgraded to any versions after 2016, SSRS reports that are used to display in iFrame will fail to display.
Solution:
Go to URL and change the URL to reportviwer, if the URL is using reports then it will not dispay in the iframe.
Example:
Change From:
https://Server/Reports/browse
Change To:
https://Server/ReportServer/Pages/ReportViewer.aspx?&rs:embed=true
or
http://Servername/ReportServer/Pages/ReportViewer.aspx?%kjhkjs%khjkhkjhy%retre+uytts%2popiopoi+iuiuyiuy&rs:Command=Render&Rs:embed=true
Note: this will be your report name -> %kjhkjs%khjkhkjhy%retre+uytts%2popiopoi+iuiuyiuy