UNREAL 5 NAVMESH NEEDS TO BE REBUILT ( IN RED ) CITY SAMPLE

HELLO,
so i’ve made a game around the city sample but it says navmesh needs to be rebuilt.

i can’t find anything online to help me, and i can’t pack my project because of it. pls give a detailed description of what to do. i see people say go to level blueprints and put in a command, but idk how to.

it over 400 hours of work, and this has stopped me dead in my tracks and i fear i will never be able to release my game because of this. pls help