What meeting victims must convince the criminals of is that their actions have effects.
What does the sentence suggest?
It suggests the person was not thinking very carefully about communicating clearly when they said/wrote it!
This is how it needs to be divided:
What meeting victims must convince the criminal of / is that their actions have effects.
I presume there is some sort of a meeting taking place between criminals and the victims of their crimes.
The victims need to persuade the criminals that the (criminal’s) actions affect the victims.