You will find the logs for SSRS on the server in the folder “%programfiles%\Microsoft SQL Server\
Here is an example below.
You may see several instances of SQL however you are looking for the one that says MSRS (You may have others there for Analysis Servers MSAS and SQL Server MSSQL )
If you need to find out more about the different kind of log files in this folder there is a table here.
Otherwise just get into it!