So I need to create a cell grid with 1km2 for each cell. Since the default WGS84 uses degrees and not meters/kilometers for unit of measurement, I tried to change the projection for one that use meters (EPSG:5017 and others Portugal based ones), and changed de CRS of the layers to the same one. That made QGIS give this error message (Error: Fallback transform failed) every time I try to move around the map (though zooming in and out still works), and I still can't create a grid because when I try to use a layer for the grid extent or try to draw it, it gives WGS84 coordinates, while the "create grid" function interprets them as EPSG:5017 coordinates (image 1). Also, the thing at the bottom that shows the coordinates of the mouse still seem to be in WGS84, so I can't input the coordinates myself.
This is getting frustrating because every time I think I found a work-around, something doesn't work the way that it should (or that I though it should) or doesn't work at all. I don't think it's a problem with the projection system it self, but it might be because I only tried systems that start with "Lisbon", but I don't what other systems could work.