System is not standard Fortran but it is defined in the module IFPORT so add USE IFPORT to your source. Better still use EXECUTE_COMMAND_LINE which is standard fortran,
The @8 indicates that you have asked the compiler to use the STDCALL calling convention, possibly as a result of migrating the application from Compaq Visual Fortran or using the /iface compiler option. Right click on the project, select Properties. Go to Fortran > External Procedures. Change Calling Convention to "Default" and String Length Argument Passing to "After all arguments".
I do agree with Andrew's recommendation of the standard intrinsic EXECUTE_COMMAND_LINE instead of SYSTEM.