You can get this error if you have a 32-bit DISCON.DLL and are using a 64-bit version of FAST or vice versa (64-bit DISCON.DLL with 32-bit FAST). If you have an NREL-supplied version of FAST 7, it should be a 32-bit application.
You can also get this error if DISCON.DLL isn’t in the current working directory. This is the directory where you are running FAST v7 (not necessarily the same place the FAST executable is located or the input files are located). You don’t need the DISCON.f90 file after you have created DISCON.DLL.
If neither of the above solutions work, check that your DISCON.DLL was compiled properly.