Weird black line

How to fix this

Already tried recalculate normals? Edit mode > Shift+N

It looks like you have non-manifold faces ( not FLAT) in the upper section.
I would try this…
Delete the faces… Select Vertices in the upper section and Fill…check if the black line comes back…if not… then select lower section vertices and upper and Connect Vertex pairs…then Fill.
You may have to subdivide the upper section a couple of times to get rid of what I think is a twist in the upper face.

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Thank you RShlers.

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