This is an error code resulting from a failure of the statement open(12,file='/users/vaidehi/xfr/Projects/2020/CDP/first-impact6.res',status='...'). If status='old' and the file doesn't exist, you will get this error. If status='new' or status='unknown' and the file can't be created, you will get this error. If the file exists and is open by another program, then you can't create it. If the folder doesn't exist, you can't open it, because you can't create a folder with this statement. If there is no room or the destination is write protected or not writeable (like a CD), you will get this error. On some systems, if the file name is too long, you will get this error. On some O/S you can't use "/" as a folder separator, you must use "\", which is an escape character "\\" as interpreted by some compilers. These last two causes are much less likely than the previous ones.