The terminology of the program can be very specific, what exactly do you mean by “Practice Interval”, or what exactly do you want to do?
Assuming all cards / subdecks are using the same deck option post, and you’re working about interval of review cards, which parameter have you set to 30 days in your deck options?
I meant the interval that cards cycle by “practice interval.”
When I grouped decks as a group, some decks had 15-day interval and some had 30-day.
And I want all cards to be 30 days interval but I’m not sure how to reset cards to set up for a new interval.
In Anki, intervals refer only to review (learned) cards, not to new (not learned) ones, and you can’t set a fixed interval for your review cards, since the main point of SRS is that the software will decide when it’s better for you to review your cards again. Having said that, you can set a max interval of 30 days, if you want. https://docs.ankiweb.net/deck-options.html#maximum-interval
For new cards, you can set a learning step of 30 days in your deck options screen, just make sure that all your cards share the same deck option preset. https://docs.ankiweb.net/deck-options.html