In my previous company I was doing cash in hand so don't have any payslip or salary cert but I have work experience cert from the employer is that enough?
But in the reference letter, it says salary depends upon the project I am working on.
All you should need in most cases is a letter from the employer outlining what your duties were and how long you worked for them. The duties need to match the NOC you're claiming experience for. Don't think they're usually interested in payslips outside of a few circumstances.