tente usar aspas duplas ""
:
grep -oP "FW_6.0.0, SUCCESS" file
OR (porque é uma string fixa, não um padrão):
grep -oF "FW_6.0.0, SUCCESS" file
da página man do grep:
-F, --fixed-strings
Interpret PATTERN as a list of fixed strings, separated by
newlines, any of which is to be matched. (-F is specified by
POSIX.)
-P, --perl-regexp
Interpret PATTERN as a Perl regular expression. This is highly
experimental and grep -P may warn of unimplemented features.