Fiverr takes 20% commission from all sellers. This is NOT a tax, but the commission Fiverr takes. It’s their cut for providing the service.
Meaning if you have an order for 100 USD; you will get paid out 80 USD.
It’s not up to Fiverr to pay your taxes, nor is it up to Fiverr to provide guidelines for this, because they are ever-changing, and varies from country to country.
When you work as a freelancer, there is a set of guidelines, regulations, and laws about this in each country; how much tax you need to pay, how you pay it, etc. This is your personal responsibility to solve, just like it is for anyone holding a normal day job.
It’s simple, really; get in touch with your local tax office, explain that you wanna work as a freelancer, and ask for information about the tax there.