If you also find error like "laravel-elixir not found" on gulp then you can install laravel-elixir npm package manually. I also fetch this error when i was new in laravel and working with gulp first time.
We can solve laravel-elixir not found error using laravel-elixir nodejs package. So you can run just bellow command and check. First command if you run already then don't require to run.
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npm install laravel-elixir