Can’t clear tile blockers

I’m new to this game so i apologize if i missed something stupid or anything like that.

I’ve had a planet colonized but it has a lot of toxic kelp. I researched the technology to remove toxic kelp, but clearing the blocker doesn’t do anything. It adds it to the building queue like normally but nothing happens. I’ve tried building new districts but nothing happens when i do that either.

Am i forgetting something or is there a bug?