I got my answer from the Discord channel of the library.
It seems that 5.x versions are not supported, and they pointed me to use 4.1.0 and that worked.
Bellow you can see the full answer
What is the FullCalendar version, their latest version has some issues. I think PrimeNG supports 4.1.0 right now, 5.x is not supported. If this is urgent, you may try the official angular wrapper of FullCalendar as well. In the future, we're planning to implement our own Scheduler component and not use FullCalendar.
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