Hi. It's not a line, it's a shadow caused by a combination of the default light and a "flat" object being "smoothed" via smoothing groups.
You can get rid of that by changing the default light. Right click on [+] upper left in the viewport, select "configure" in the roll out, now change default light from 1 to 2 (or opposite). The shadow should be gone now. If not select the faces/polys (sub object level) and clear all smoothing groups (placed near the bottom of the editor roll out).
Hope it helps
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3dsMax 4.2 to 2018
AutoCAD 2000 to 2018
Infrastructure Design Suite Premium 2012 to 2018