Synthia does not accept phosphazine derivative as a repeat unit, please help?

So as seen in the uploaded picture, Synthia does not accept this structure as a valid repeat unit

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Can anyone please tell me what I am doing wrong and also what can be done to resolve this problem? I understand that this unit is a dendrimer but I was not able to add it in the form of dendrimer (using build command in Material Studio) to Synthia.