This issue was caused when the reference data was migrated to the new MO_REGISTRATIONS table.
We can provide a script to solve this, Please raise a case and reference the title of this article.
Registrations no longer available for Adult records and category required for child records
In this article, we explain why categories may be required, adult registrations may be empty, and new ones can't be added.
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Written by Yusef Abulaynain