The instructor explains how the fire action is mapping to the left mouse button. At the top of this window in bright yellow, there is a warning from Unreal saying this is now deprecated and to instead use Enhanced Input Actions or Input Mapping Contexts. Could you please show us how to use these instead so that the material we are learning is up to date and relevant.
They have been flagged as deprecated since 5.1 so probably by 5.5 it will be disabled. Right now, follow the course as is but feel free to explore the new input system.