If we put an extra sensor next to the slits, we will be able to measure if the electron went through the left slit or through the right slit. We will not detect the electron in both slits because the electron cannot split into half-electrons. So, the amplitude will be either fully in the left slit, and zero in the right slit, or fully in the right slit and zero in the left slit. There will be no interference of the outcomes from the left and right slits (as it will always be zero in one of the slits). Therefore, after shooting many electrons, we will see two bright spots on the screen.
If we don’t measure the outcomes of the sensors on the screen then we don’t demand an electron to be detected on either left or right screen. The amplitude does not have to be fully one in one of the slits, and zero in another. The amplitude can go through both slits and the outcomes from each slit can interfere, and create the interference pattern on the screen.