write a simple scanner and parser in lex and yacc

Need to write scanner in lex and parser in yacc to the language described in the pdf. The parser should accept any possible code in the described language and build and AST (abstract syntax tree) if there is an error. (Token or syntax error). The parser should print error message based on the error type and explain exactly what the error is. You have to write a function that scans the AST tree in preorder and prints the AST if the code is valid.