We’re talking different things here. If you manually enter an intercept heading, it will draw a straight legs extension that may or may not be interceptable depending on distance to the path and intercept angle. If you don’t enter an intercept heading it will draw a turn that we clean up by redirecting once established on the turn.
If these conditions aren’t met you’ll get the “NOT ON INTERCEPT HEADING”. The course of action is usually to use HDG SEL to establish yourself on an intercept heading and then arm LNAV.
