Hi Max! You are fighting against grid snap and the difficulty of scaling in two directions at once. Fortunately, this map looks pretty simple to correct. Go through these steps and you should be right as rain. Snap the map to the upper left grid corner. (OPTIONAL—if your map actually is already cropped to be exact squares, grid snap may not be na issue. If you have partials squares at the edges, you may need to turn off snap). Right click on the map and choose "Advanced > is Drawing". This will turn off snap for the image. Alternatively, you can just hold down alt/opt while scaling to temporarily avoid snap. Scale from one of the center handles. do left to right first. Grab the handle on the right-hand-middle and stretch it horizontally until the grids match. Don't yet worry about vertical. Once the map squares line up horizontally, repeat the previous step, but grab the bottom-middle handle to only scale vertically. Once those line up, your map should be aligned on both axes. Don't worry if the Roll20 grid does not line up precisely with the printed grid. This is a possibility depending on how the map was generated. If there is a few pixel discrepancy here and there, open your page settings and bring the opacity of the grid to zero, and just rely on the pre-printed grid. No one will care (or likely even notice) if the tokens don't exactly snap to the very edge of the grid.
