Ok so, try inputting using on-screen keyboard on the EFB rather than your keyboard. And after setting it, try restarting sim. Sometimes if SimConnect has died, it stops loading.
Other things to try is a different flightplan (make sure the official simbrief profile is used, linked in FAQ)
This error also happens when route is invalid for the current AIRRAC.
If you're using an outdated AIRAC to generate the flightplan on Simbrief, but sim uses latest by default it'll cause this error.