You have tried to write to a file in sequential access mode that you have not opened for OUTPUT or EXTEND, or you have tried to write to a file in random or dynamic access mode that has not been opened INPUT or I-O, or which is not open at all.
Close the file and reopen it with the correct open mode for the file type. However, as this error implies that your program logic contains a mistake, you might want to close any files that are open, execute a STOP RUN statement and then recode.