DIAGNOSIS The streaks are from dust, fingerprints, or dried smudges on either the flat glass you place the document on or, more commonly, the underside of the scanner lid (which often has a white backing sheet that gets dirty).
FIX Power off the printer. Lift the scanner lid. Use a can of compressed air to gently blow any loose dust off the glass and the white backing on the lid. For fingerprints or smudges, put a small amount of distilled water or isopropyl alcohol (90% or higher) on a microfiber cloth—never spray directly onto the glass. Wipe the glass in one direction, not circles. Use a dry part of the cloth to immediately buff it dry. For the white backing on the lid, wipe gently with a dry cloth first; if needed, use a barely damp cloth and be careful not to saturate it.
IF THAT DIDN'T WORK If streaks are still appearing in the same place on every scan, there might be a tiny scratch on the glass or a permanent stain on the backing sheet. You can test which it is by scanning with the lid open and a blank sheet of white paper placed over your document. If the streak disappears, the issue is the lid's backing sheet.
HEADS UP Never use window cleaner, acetone, or other abrasive chemicals. They can strip anti-reflective coatings. Also, avoid paper towels or tissues—they can scratch the glass. The glass is more fragile than it looks, so don't press hard.