I have 2 subgrids on a form in Dynamics 365 subgrid one is my People list which is set up exactly as I want with the Add Existing Button. Subgrid 2 is my Waiting list and I have it set up exactly the same as People with a N to N relationship although I dont see a Add existing or an add new button.
1. Please check if the Field Requirement of the lookup field of the N:N relationship is Business Required.
Please refer to this blog:
2. Please check if this entity is set to read-only in mobile. Also, make sure your security role has permission to create this entity.
3. Use the Ribbon Workbench to check if the button has an associated Display Rules or Enabled Rules.
Please refer to this post for details:
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