>>304929112
In blender 2.8x select those vertices with like a block select or something (b) in see through mode (wireframe usually is) then f3 and search for merge by distance. basically there is a seam because the vertices aren't connected. the 2.79 version is merge doubles. you can change the merge distnace on the dialogue that come on the upper left. if there's a seem just because two connected meshes were split up then the default should be good. if you're frankensteining stuff together you might have to bump it up a little
make sure to back up and save to a new blender file after doing something like this in case you fuck it up