As the others have noted, you must copy and paste here the actual commands you entered and Oracle's response so we can help.
Oracle updates the headers of data files with SCN's, and to recover, those need to be synchronized with the control file via redo information that has been archived. If Oracle can't figure it out, in some cases it will tell you the system data file is the problem, when it really means to tell you the problem data file can't be synchronized with the system data file.
DBA work requires precision, especially with complicated procedures such as manual recovery. Do things step by step, and record exactly what happens so you can check later. It isn't rocket science, but it isn't watching television either.