You can send a full Series Volume to print early, even if you don't have a credit available. You'll be charged the same price as a credit, which depends on the size of your Series.
How do I send a Series book to print without a credit?
Go to your active Series
Once a Volume has 60 pages, tap into it and tap Send to print at the bottom of the screen
When asked "Want to send your book to print early?", tap Continue
Review your order details, then tap Place order
Learn more about how Series credits work in How does a Series subscription work?