I have a booking through Open Table for the Mythos in Oct very easy to do. But I can't book Mama Della's Ristorante it keeps telling I am to early but doesn't say when to do it??

I just checked on that today too. We are going at the end of the month and I only saw late afternoon times. Maybe calling would work better. I might try that. I wonder if walk up reservations are available.
Calling will generally just get you an answering machine. There are only 4 restaurants in the Universal parks that take reservations, so there isn't somebody manning the phone at all times.
They do take walk up reservations and if you are staying onsite, you get priority seating.
I think Mama Della's is 90 days.
The times are limited because it closes well before the park does unless it is peak season and even then it is not open late.During slower times of year it is not unusual to have the latest ressies for 330-4pm
I wonder if walk up reservations are available.