You did mention that you use to sign a cash voucher before taking your salary. Those cash vouchers can and are usually treated as pay slips. You should ask your employer for a copy of that cash voucher(s).
See, for Express Entry, the requirement is that you should be working full time, for which your employment/offer letter should suffice and and that your work should had been paid work, for which a bank statement showing regular deposits from the same person/company and the same/similar amount could be used as evidence, since you were paid in cash, you do not have such bank statements to prove your case, in the case, you would require copy of those cash vouchers as proof of payments made to you.
What really matters is whether you can prove that you worked there, such as a letter of recommendation, pay stubs, or even the cash voucher you mentioned. Many people face this, especially in small firms. While researching similar cases, I came across
https://icasinoreviews.co.nz/ where, surprisingly, there were also some useful discussions on similar payment topics. I love playing games, so this is exactly what I needed. Just make sure your paperwork is solid and meets the NOC requirements. You have it - just be honest and detailed in your application!
See, it is not illegal to accept cash payments as long as cash is declared and you pay your taxes according to the tax slabs you fall into. However, you need proof that you were being paid in Cash and you received those payments in cash and for that you need either salary/pay slips issued by your employer or those cash vouchers you had signed while receiving cash. Ask your employer to generate and give you salary slips or to give you copies for signed cash vouchers.