After a brief look at the main.nf it does appear to still be using DSL1. You could try adding nextflow.enable.dsl = 1 to the script to see if it fixes the error. Another option is upgrading the script to DSL2, which might be tricky dependent on your comfort with Groovy. Finally, you could check your version of nextflow to ensure you are using Nextflow 22.10.x or earlier (you can use conda list nextflow or nextflow -version to check this while your environment is activated).

See this part of the nextflow documentation for more information on DSL migration.

DSL1 is not supported anymore, but you can always download+compile the NF that was released at the time of your DSL1 workflow : https://github.com/nextflow-io/nextflow/releases

As the others have said, you need an older version of nextflow which still runs DSL1 (think v1 of nextflow) scripts. The current Nextflow DSL version is DSL2.

To fix this (if you have java installed) you can do this.

wget https://github.com/nextflow-io/nextflow/releases/download/v23.04.4/nextflow-23.04.4-all
mv nextflow-23.04.4-all nextflow
./nextflow

Or if you prefer conda you can just make sure you get the 23.04 version of nextflow to make sure it runs.

Sometimes java versions and nextflow versions don't get along too well either. I used to use Java 11.0.1 but know use Java16 successfully with the complete distribution, ie. listed in the wget command above.