Sometimes, when you downloading a map, the map has line or noise errors. You can fix it using a plugin named Fix Landsat 7 Scanline Errors.

In the tutorial, clipping mask is used to make Scanline Error Processing works faster (it takes while to finish the whole scene for a small spec. laptop). Skip it if you need the whole scene.

Here is the link to download the toolbox:
https://drive.google.com/file/d/15sc0...