Nextflow offers a series of free Fundamentals Training: https://training.nextflow.io/basic_training/
A Nextflow Carpentry course is being developed: https://carpentries-incubator.github.io/workflows-nextflow/instructor/01-getting-started-with-nextflow.html
A biocommons workshop is also available: Learning how to write basic Nextflow pipelines: https://www.biocommons.org.au/events/hello-nextflow