How to generate a parser with PSI classes via Generate Parser Code from the context menu ?

3.4. Generate a parser

Now when the grammar is defined we can generate a parser with PSI classes via Generate Parser Code from the context menu on Simple.bnf file. This will generate a parser and PSI elements in gen folder. Mark this folder as Generated Sources Root and make sure everything is compiled without errors.

https://www.jetbrains.org/intellij/sdk/docs/tutorials/custom_language_support/grammar_and_parser.html#generate-a-parser