Firstly, I apologize because this question has been asked already and I can’t seem to find the post.
I have had several occasions where I load a Simbrief plan without a STAR added in. When I select my arrival runway and select the STAR I want in the FMC, I end up getting a discontinuity. I try to CLR it and I get the error NOT ALLOWED.
I start over and I select the arrival runway and then do not select a STAR, and everything loads properly.
Why can I not remove the discontinuity?
Does the STAR in question include “EXPECT RADAR vectors to…”?
I’ve seen this on one of my flights. I departed (it let me depart) with the discontinuity and when close to the last fix in the STAR, I hit direct to the first fix in the approach. This worked. It was the IRONS 7 arrival into KDCA (I know, probably not an airport you’d expect to see an A310 but anyway).
Not sure. I was trying to set it up on the ground before take off, which is unconventional I realize.
What would be the reason for the “not allowed” error when trying to remove the discontinuity?
I don’t know the correct answer. In my case I assumed it was because radar vectors from the controller would be an unknown quantity for the FMC.
Which STAR were you entering?
While this explainer is for the FBW A320, it gives a good primer on discontinuities in general, and is worth a read. My guess is what has been suggested above, it’s probably a STAR that requires radar vectors, something that MSFS doesn’t yet support in its ATC simulation.
Thanks for that. Read the section of the manual that you linked. Learned something 
Learn something new every day! There was, indeed, a vector waypoint prior to the discontinuity in my situation.
From this I’m gathering two things…remove the vector waypoint, or more realistically, do a DIR TO operation in flight to the next waypoint that will be entering into the STAR.
The most realistic way to handle a discontinuity for vectors is to fly your own headings to intercept final (assuming you don’t have access to realistic ATC.)
Blockquote The most realistic way to handle a discontinuity for vectors is to fly your own headings to intercept final (assuming you don’t have access to realistic ATC.)
Basic newbie question: When I get to a discontinuity, I do what is suggested above – I’ll fly my own heading (by hitting the heading button to disengage that aspect of the autopilot) and manually fly to the discontinued waypoint. I’ll then manually head toward the next waypoint and try to re-engage the autopilot’s control of the heading. But hitting the heading button again doesn’t seem to work for this. Is there generally a way this should be done?
Thanks.
Generally speaking, you would do a DIRECT TO to the waypoint in the flight management computer, then re-enable NAV mode and it should resume GPS navigation.
This procedure is common for many planes in the sim.
