The tank in the Steel Tide mission isn’t usable, upon turning in either direction it turns the full way and kicks you off the tank.
The Steel Tide Tank BugSource
https://forum-en.guildwars2.com/forum/game/story/The-Steel-Tide-Bugged/1177665
In the meantime, you can also use the nearby mortars to fight the Risen. The objective is based on killing X giants.
