The as command requires permission to open the indicated file with the indicated permission.
In the message text:
string1 is the processing file name. string2 contains the file attributes passed into fopen(). number is the errno set by fopen(). string3 is the error message associated with errno.
If the indicated file already exists on your system, rename the file and reenter the as command. Review the fopen() information in the XL C/C++ Run-Time Library Reference and use the errno to determine the cause of the error.