The error message says the JDBC driver doesn’t support setString.
What version of the postgres JDBC driver are you using? There may be a later version that does support this.
UPDATE:
Looking at the Java source file for the JDBC driver, this is not supported in the Postgres JDBC driver. It is coded to return that error message if used.
https://github.com/pgjdbc/pgjdbc/blob/master/pgjdbc/src/main/java/org/postgresql/jdbc/PgCallableStatement.java#L729