Hi,
Are you using the MSFS version of the aircraft? I believe flight plan importing is normally accomplished by entering your SimBrief ID into the settings, rather than by downloading the route file.
It might still be possible to download the files as well though. This thread seems to discuss importing methods: https://www.reddit.com/r/MicrosoftFlightSim/comments/1gadejz/ifly_737_max_8_simbrief_integration
Best regards,