It is with the sea zones that have changed post-RoI, right?
The tutorial advises you to take a sea province and use it as your land province and then add a new sea province at the end.
Now you can just pick the number that max_provinces is set to and increment max_provinces by one. No need to move around an existing province.
Then you'd have to add the localisation for the province, edit landed_titles, add history/provinces, add a flag and you should be good to go.
Edit. I forgot definitions.csv. Very important.
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