What you are quoting does not mean that an economy ticket has to be issued 21 days in advance.
Nor did I suggest that it did. It says what it says. But the OP has already waited much too long to get an initial booking into the system. If she is already inside of three weeks until her (intended) departure, then obviously that is even worse. She needs to make a reservation asap. And depending on the exact date of her first segment, she needs to be aware of the ticketing rules.
cheers,
Henry