We apologize for this issue and look to get it remedied in our next update.
There are multiple additional Coalition enclaves created at the start of the mission. These enclaves are not marked by the mission, but are named "Coalition" on the map. Doing the following has resolved the issue for other players:
1) Restart "Warlord: Smash the Coalition" by restarting the game or switching characters;
2) Open your map screen when the mission starts again;
3) Eliminate all extra hostile Coalition enclaves before traveling to the one marked by the mission;
4) Travel to the marked location and eliminate the final Coalition enclave there.
Please comment below to let us know whether this helps you complete the Warlord legacy.
Thanks for playing State of Decay 2!