Just to complete the diagnosis for those who may search and read this thread later and still be confused...
In a Yaschica Mat 124G TLR the 120 roll film travels vertically, thus the images are oriented vertically. When that same film is tank-developed it is usually oriented horizontally after being wound onto spiral reels. This places one vertical edge of the images at the top of the tank.
In a tank with insufficient developer solution that vertical edge is now exposed above the developer for the full length of the film. It only gets temporarily submerged when the tank is inverted for agitation, so it does not develop fully.
When the film is then placed in the fixer that underdeveloped strip is then dissolved away more completely, thus leaving a relatively more transparent strip across that edge of the negative.
This more transparent edge then prints (or scans) as a darker band, since more light can now pass through it. The wavy lines of differing density within the darker band result from sloshing inside the tank after agitation, before the liquid eventually settles down and becomes still again between agitation cycles.
Sorry for the long-winded details, but it's almost a sure thing that someone else brand new to film developing with the same problem will search and arrive here, and not necessarily be able to fully visualize what actually happened.
Ken
(Note to the original poster: It might also be nice to request the moderators to rename this thread to something more search-friendly. Like maybe
"Help! Dark bands across my picture edges!" or something equally as dramatic...