you are missing the actual program texlab as the message mentions. you didn’t say what happened after answering “yes” but chances are it will fail as the download texlab executable wont work.
add pkgs.texlab to your environment.
We’ll try to make it work out of the box in the future see collect and add to PATH runtimeinputs of neovim plugin dependencies · Issue #352738 · NixOS/nixpkgs · GitHub