Random opening that can't edited/ patched, preventing the mesh to be mirrored?

Hey everyone, continuing on with the awesome helmet tutorial! Here I am running into a very interesting problem where there seems to be a random hole by the side of the mesh that I cannot select to edit and patch. it also looks like I cannot mirror this sheet over as well. Could this be some sort of internal reason as to why this can't be mirrored? Love to hear your thoughts and insights. Thanks!

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