You can do it using a SQL (command STRSQL) like this :
Update FILE set FIELD = ‘ ‘ where SOMEFIELD = keyvalue
FILE = your file
FIELD = your field or fields in the unique key.
SOMEFIELD = the field you want to clear.
keyvalue = the new value (zeros or blanks, it depends of the type of your field).
The not found indicator maybe is because you don’t have the K(eyed) value in the file declaration row. This is an example>
FSOMEFILE if E K DISK
If you don’t put the K letter, the keyed file return a not found indicator when you do a chain.
Hope this help you!