You're not doing anything wrong. You're just finding the limits of the camera/lens combination you are using.
- The EF 50mm f/1.8 (in various versions) has been known as the "plastic fantastic" for a long time. For what it can do at what it costs, it is a fantastic value. But it isn't really a fantastic 50mm prime lens when compared to many others that, admittedly, cost a little to a lot more. One of the weaknesses of the EF 50mm f/1.8 II is that it has fairly large steps between each focus position when compared to other Canon lenses. In fact, it has the largest steps of any Canon EF lens on the market. This means if you are trying to focus on something that is in between one step and the next, the camera has to decide whether to go slightly past your subject or slightly in front of your subject. Another weakness of the EF 50mm f/1.8 family is that they need to be stopped down a bit to reach their maximum center sharpness. Wide open at f/1.8 even the center of the image is a little soft compared to when the lens is stopped down a few clicks.
- Your Canon EOS Rebel T6/1300D is another piece of Canon gear that is a pretty good value but the low price comes with the caveat of giving up some of the things higher priced cameras offer. One of those features is AFMA (Autofocus Micro-Adjustment) that allows the end user to calibrate their cameras AF system to a particular lens. If a lens consistently misses in front of the desired subject, AFMA can be used to tell the camera to focus a little further than what it otherwise would.
If you are shooting from a tripod there is a way to increase the accuracy of your camera's AF, but it comes at the cost of slower AF and using the rear LCD in Live view to compose your shot instead of being able to use the optical viewfinder. When you are shooting in Live View the camera uses contrast based autofocus from the signal it is receiving from the main imaging sensor. When you shoot through the viewfinder the camera uses a dedicated phase detection AF sensor because the mirror that reflects the scene into the viewfinder blocks light from reaching the main imaging sensor.
Another thing you can try to do is stop down to f/2.2 or f/2.8 where center sharpness increases significantly from f/1.8. At f/2 the lens is a little better than wide open, but it's still noticeably softer than even narrower apertures.