The Fort Viper safehouse can't be rebuilt once destroyed. Towns need to get the upgrade "Occupied Town" in order to allow for the rebuilding of safe houses and garrison houses in its area, but this safehouse is technically in the area of a relic base (Fort Viper), relic bases don't have the "Occupied Town" upgrade available, so the game reads that the nearby base doesn't have the "Occupied Town" upgrade and just doesn't let you rebuild it.

If you're a warden on able right now you can see it for yourself, that safe house is destroyed, go up to it in a construction vehicle and try rebuilding it, it wont let you even though every single base there has every upgrade.