That mate won't work because both Points belong to the upper level Assembly. In order for that to work at least one of them, and preferably both, need to be in the Part file. You can mate an entity belonging to a Part or Assembly to an entity belonging to any other Part or Assembly, but not two belonging to the same Part or Assembly.
In any case, I'd suggest a different workflow, and use a Profile Center mate instead. That will accomplish your goal without the need for added reference geometry, and it can be set to allow rotation, or you can lock it.
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